[OpaqueRange] Introduce NodeRange interface

Add NodeRange between AbstractRange and Range/StaticRange. Under the
OpaqueRange flag, startContainer/endContainer move from AbstractRange to
NodeRange, allowing OpaqueRange to inherit from AbstractRange without
exposing internal DOM containers.

Keep startContainer/endContainer on AbstractRange behind the inverse
LegacyAbstractRange flag until OpaqueRange ships. Add the C++ NodeRange
parent unconditionally so the inheritance chain stays stable.

Relevant discussions: whatwg/dom#1470
whatwg/html#11478 (comment)

Signoff from Blink API Owners on change:
https://groups.google.com/a/chromium.org/g/blink-dev/c/otEJ9AacRg4

Low-Coverage-Reason: COVERAGE_UNDERREPORTED
Bug: 421421332
Change-Id: Ica8f37cc536044269f1e252b63c38e7c8b55e758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7904930
Reviewed-by: Mason Freed <masonf@chromium.org>
Reviewed-by: Dan Clark <daniec@microsoft.com>
Commit-Queue: Stephanie Zhang <stephanie.zhang@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1663534}

Author: stephanieyzhang

dom/ranges/tentative/OpaqueRange-basic.html

@@ -18,8 +18,8 @@
   const textarea = document.body.firstElementChild;
   const range = textarea.createValueRange(1, 4);
 
-  assert_equals(range.startContainer, null, "startContainer should be null.");
-  assert_equals(range.endContainer, null, "endContainer should be null.");
+  assert_equals(range.startContainer, undefined, "startContainer should be undefined.");
+  assert_equals(range.endContainer, undefined, "endContainer should be undefined.");
   assert_equals(range.startOffset, 1);
   assert_equals(range.endOffset, 4);
   assert_false(range.collapsed);

dom/ranges/tentative/OpaqueRange-disconnect.html

@@ -13,8 +13,8 @@
   assert_equals(range.startOffset, 0, "startOffset resets to 0");
   assert_equals(range.endOffset, 0, "endOffset resets to 0");
   assert_true(range.collapsed, "range is collapsed");
-  assert_equals(range.startContainer, null, "startContainer remains null");
-  assert_equals(range.endContainer, null, "endContainer remains null");
+  assert_equals(range.startContainer, undefined, "startContainer remains undefined");
+  assert_equals(range.endContainer, undefined, "endContainer remains undefined");
   assert_equals(range.getClientRects().length, 0, "no client rects");
   assert_equals(range.getBoundingClientRect().width, 0, "bounding rect width is 0");
   assert_equals(range.getBoundingClientRect().height, 0, "bounding rect height is 0");

dom/ranges/tentative/OpaqueRange-range-updates.html

@@ -23,16 +23,15 @@
 
   // Get range on <input>.
   const inputRange = input.createValueRange(0, 4);
-  // Opaque range: containers are null.
-  assert_equals(inputRange.startContainer, null);
-  assert_equals(inputRange.endContainer, null);
+  assert_equals(inputRange.startContainer, undefined);
+  assert_equals(inputRange.endContainer, undefined);
   assert_equals(inputRange.startOffset, 0);
   assert_equals(inputRange.endOffset, 4);
 
   // Get range on <textarea>.
   const textareaRange = textarea.createValueRange(0, 5);
-  assert_equals(textareaRange.startContainer, null);
-  assert_equals(textareaRange.endContainer, null);
+  assert_equals(textareaRange.startContainer, undefined);
+  assert_equals(textareaRange.endContainer, undefined);
   assert_equals(textareaRange.startOffset, 0);
   assert_equals(textareaRange.endOffset, 5);
 }, "createValueRange works on different form controls.");
@@ -46,8 +45,8 @@
 
   // Verify disconnected <input>.
   const inputRange = input.createValueRange(0, 12);
-  assert_equals(inputRange.startContainer, null);
-  assert_equals(inputRange.endContainer, null);
+  assert_equals(inputRange.startContainer, undefined);
+  assert_equals(inputRange.endContainer, undefined);
   assert_equals(inputRange.startOffset, 0);
   assert_equals(inputRange.endOffset, 12);
 

dom/ranges/tentative/OpaqueRange-supported-elements.html

@@ -18,9 +18,8 @@
 
     const range = element.createValueRange(0, value.length);
 
-    // Opaque range: containers are null.
-    assert_equals(range.startContainer, null, 'startContainer should be null');
-    assert_equals(range.endContainer, null, 'endContainer should be null');
+    assert_equals(range.startContainer, undefined, 'startContainer should be undefined');
+    assert_equals(range.endContainer, undefined, 'endContainer should be undefined');
     assert_equals(range.startOffset, 0);
     assert_equals(range.endOffset, value.length);
     assert_false(range.collapsed);