Fix -webkit-image-set serialization

The '-webkit-' prefixed 'image-set' is expected to serialize to the
same value as standard 'image-set'.
https://drafts.csswg.org/css-images-4/#deprecated
"Implementations must accept -webkit-image-set() as a parse-time alias
of image-set(). (It’s a valid value, with identical arguments to
image-set(), and is turned into image-set() during parsing.)"

With this change Chrome will match the spec as well as Firefox and
Safari.

R=pdr

Bug: 630597
Change-Id: I95fe2a10815afd8e3169299b0f33e688950c5ce2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4167006
Reviewed-by: Philip Rogers <[email protected]>
Commit-Queue: Traian Captan <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1094022}

css/css-images/image-set/image-set-parsing.html

@@ -11,6 +11,13 @@
 function test_valid_value_variants(property, value, serializedValue) {
   if (!serializedValue) serializedValue = value;
   test_valid_value(property, value, serializedValue);
+
+  // The '-webkit-' prefixed 'image-set' is expected to serialize to the same
+  // value as standard 'image-set'.
+  // https://drafts.csswg.org/css-images-4/#deprecated
+  // "Implementations must accept -webkit-image-set() as a parse-time alias of
+  // image-set(). (It’s a valid value, with identical arguments to image-set(),
+  // and is turned into image-set() during parsing.)"
   test_valid_value(property, "-webkit-" + value, serializedValue);
 }