Media Capabilities: Add robust WPT for encryptionScheme plumbing

Previously, WPT tests for mediaCapabilities.decodingInfo()
encryptionScheme plumbing succeeded even if the plumbing was
missing or incomplete. This was because the tests only validated
that the returned object has supported/smooth/powerEfficient as
booleans using the standard Clear Key system (which supports both
cenc and cbcs).

To make these tests fail when the plumbing is missing or broken:
1. Define a helper function in WPT that queries decodingInfo() with a
   specific encryptionScheme, and asserts that the negotiated
   configuration matches the expected scheme and support status.
2. Run test cases covering different valid encryption schemes ('cenc',
   'cbcs', 'cbcs-1-9'), as well as unrecognized ('foobar') and null
   schemes. If the plumbing is missing, unrecognized schemes will be
   incorrectly supported, and valid schemes will negotiate as null
   instead of their expected scheme, causing the tests to fail.
3. Remove the obsolete and non-assertive tests that only checked
   that the result type was a boolean.

Bug: 498284510
Change-Id: I88b3c01f7b3f200312b948f18714707ca2aa7c32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8076778
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Sangbaek Park <sangbaekpark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1662185}

Author: beback4u

media-capabilities/decodingInfoEncryptedMedia.https.html

@@ -259,7 +259,7 @@
   }));
 }, "Test that decodingInfo with type webrtc rejects key system configuration.");
 
-promise_test(async t => {
+async function runEncryptionSchemeTest(t, scheme, expectedSupported) {
   const config = {
     type: 'file',
     video: minimalVideoConfiguration,
@@ -268,36 +268,54 @@
       keySystem: 'org.w3.clearkey',
       video: {
         robustness: '',
-        encryptionScheme: 'cenc'  // <- New field being tested
+        encryptionScheme: scheme
       }
     }
   };
 
-  const result = await navigator.mediaCapabilities.decodingInfo(config);
+  const info = await navigator.mediaCapabilities.decodingInfo(config);
+  assert_equals(info.supported, expectedSupported, `supported should be ${expectedSupported}`);
 
-  // As long as it resolves to an object with `supported` (even if false),
-  // the IDL parser correctly digested `encryptionScheme`.
-  assert_equals(typeof result.supported, 'boolean', "supported should be a boolean");
-  assert_equals(typeof result.smooth, 'boolean', "smooth should be a boolean");
-  assert_equals(typeof result.powerEfficient, 'boolean', "powerEfficient should be a boolean");
-}, "decodingInfo() accepts KeySystemTrackConfiguration with encryptionScheme");
+  if (expectedSupported) {
+    assert_not_equals(info.keySystemAccess, null, "keySystemAccess should be present in the result.");
+    const negotiatedConfig = info.keySystemAccess.getConfiguration();
+    assert_true(
+      !!negotiatedConfig.videoCapabilities && negotiatedConfig.videoCapabilities.length > 0,
+      "videoCapabilities should be present in the negotiated configuration."
+    );
 
-promise_test(async t => {
-  const config = {
-    type: 'file',
-    video: minimalVideoConfiguration,
-    audio: minimalAudioConfiguration,
-    keySystemConfiguration: {
-      keySystem: 'org.w3.clearkey',
-      video: {
-        robustness: '',
-        encryptionScheme: null  // <- Explicitly testing nullable
-      }
-    }
-  };
+    // If the encryptionScheme is not passed from mediaCapabilities.decodingInfo()
+    // to the EME query, the negotiated keySystemAccess configuration will not contain
+    // the requested encryptionScheme (it will be null/unspecified).
+    assert_equals(
+      negotiatedConfig.videoCapabilities[0].encryptionScheme,
+      scheme,
+      `negotiated encryptionScheme should match requested scheme: ${scheme}`
+    );
+  } else {
+    assert_equals(info.keySystemAccess, null, "keySystemAccess should be null when not supported.");
+  }
+}
+
+promise_test(t => {
+  return runEncryptionSchemeTest(t, 'cenc', true);
+}, "decodingInfo() supports 'cenc' encryptionScheme and echoes it back.");
+
+promise_test(t => {
+  return runEncryptionSchemeTest(t, 'cbcs', true);
+}, "decodingInfo() supports 'cbcs' encryptionScheme and echoes it back.");
+
+promise_test(t => {
+  return runEncryptionSchemeTest(t, 'cbcs-1-9', true);
+}, "decodingInfo() supports 'cbcs-1-9' encryptionScheme and echoes it back.");
+
+promise_test(t => {
+  return runEncryptionSchemeTest(t, 'foobar', false);
+}, "decodingInfo() with unrecognized encryptionScheme returns supported: false.");
+
+promise_test(t => {
+  return runEncryptionSchemeTest(t, null, true);
+}, "decodingInfo() with null encryptionScheme returns supported: true and echoes null.");
 
-  const result = await navigator.mediaCapabilities.decodingInfo(config);
-  assert_equals(typeof result.supported, 'boolean');
-}, "decodingInfo() accepts KeySystemTrackConfiguration with null encryptionScheme");
 
 </script>