Preserve invalid selectors in :is() and :where()

As per the resolution in w3c/csswg-drafts/issues/8356 [1], forgiving
selector lists (argument of :is(), :where()) need to preserve invalid
arguments for serialization, even though they never match anything.

Use the existing pseudo type kPseudoUnparsed with kNone nesting type
to keep an invalid selector text and change the css selector parser
so it creates the placeholder for the failed-parsing complex selector.

To keep the current First()/Next() behavior of CSSSelectorList, the
methods skip the unparsed-invalid selector.

FirstIncludingUnparsedInvalid() and NextIncludingUnparsedInvalid()
are added in CSSSelectorList to serialize with the unparsed-invalid
selectors.

Add a private method IsInvalidWithoutUnparsed() to CSSSelectorList
to distinguish an empty list (CSSSelectorList::Empty()) from an
invalid list that has unparsed-invalid selectors. Replace the IsValid()
checks that meant "empty" with it (e.g. in Copy() and ComputeLength()),
so the methods can handle the preserved unparsed invalid selectors.

The invalid selector serialization works behind the experimental
feature SerializeInvalidSelectorsInForgivingSelectorList.

[1] w3c/csswg-drafts#8356

Bug: 40277472
Change-Id: I5d350d815153fbf846ed8763707d0938a2327376
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8068106
Commit-Queue: Byungwoo Lee <blee@igalia.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1669314}

Author: byung-woo

css/selectors/parsing/parse-is-where.html

@@ -16,7 +16,7 @@
       if (valid) {
         test_valid_selector(selector, expected_selector);
       } else {
-        test_valid_forgiving_selector(selector);
+        test_valid_forgiving_selector(selector, expected_selector);
       }
     }
   }
@@ -43,6 +43,15 @@
 
   assert_valid(true, ":not({}(div))", null, "Nested inside :not, without combinators");
   assert_valid(true, ":not({}(div .foo))", null, "Nested inside :not, with combinators");
+
+  assert_valid(false, "{}(,,,)", "{}(, , , )", "Empty text, whitespace after comma");
+  assert_valid(false, ":host({}(,,,))", ":host({}(, , , ))", "Empty text, whitespace after comma");
+
+  assert_valid(false, ":host({}(.a, .b+.c, .d))", null, "Nested inside :host, serialize invalid selector as-is");
+  assert_valid(false, "{}(::before:HOVER, .a)", null, "Pseudo-element inside, serialize invalid selector as-is");
+  assert_valid(false, ":has({}(:has(.a+.b)))", null, "Nested has, serialize invalid selector as-is");
+
+  assert_valid(false, "{}(::before:HOVER  )", "{}(::before:HOVER)", "Invalid selector, trim trailing whitepaces");
   // This should be invalid even with the forgiving behavior because
   // `<any-value>` doesn't allow bad tokens:
   // https://drafts.csswg.org/css-syntax-3/#typedef-declaration-value

css/support/parsing-testcommon.js

@@ -50,8 +50,9 @@ function test_invalid_value(property, value) {
     }, "e.style['" + property + "'] = " + stringifiedValue + " should not set the property value");
 }
 
-function test_valid_forgiving_selector(selector) {
-  test_valid_selector(selector, selector, { onlyWhenForgiving: true });
+function test_valid_forgiving_selector(selector, serializedValue = null) {
+  test_valid_selector(selector, serializedValue ? serializedValue : selector,
+                      {onlyWhenForgiving: true});
 }
 
 // serializedSelector can be the expected serialization of selector,