Support ignoring charset when parsing URLs in CSS behind a flag

This is to allow testing of the desired behavior, and it's a possible
cause of an issue being discussed in Sass:
sass/dart-sass#568

There are two call sites for CSSParserContext::Charset(), one for the
regular parsing path, and one for parsing registered property values.
Rather than touching both, just throw away the charset override when the
feature is enabled.

Bug: 1485525

Change-Id: I3da69b4156d6a84bcc8e0517c954a79b522a9ec9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4846986
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1200109}

third_party/blink/renderer/core/css/parser/css_parser_context.cc

@@ -19,6 +19,7 @@
 #include "third_party/blink/renderer/core/loader/document_loader.h"
 #include "third_party/blink/renderer/core/page/page.h"
 #include "third_party/blink/renderer/core/permissions_policy/layout_animations_policy.h"
+#include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 
 namespace blink {
 
@@ -175,9 +176,12 @@ CSSParserContext::CSSParserContext(
       use_legacy_background_size_shorthand_behavior_(
           use_legacy_background_size_shorthand_behavior),
       secure_context_mode_(secure_context_mode),
-      charset_(charset),
       document_(use_counter_document),
-      resource_fetch_restriction_(resource_fetch_restriction) {}
+      resource_fetch_restriction_(resource_fetch_restriction) {
+  if (!RuntimeEnabledFeatures::CSSParserIgnoreCharsetForURLsEnabled()) {
+    charset_ = charset;
+  }
+}
 
 bool CSSParserContext::operator==(const CSSParserContext& other) const {
   return base_url_ == other.base_url_ && origin_clean_ == other.origin_clean_ &&

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -134,7 +134,7 @@
       default: "",
     },
 
-    // Specifiy the default value of the base::Feature instance. This field
+    // Specify the default value of the base::Feature instance. This field
     // works only if base_feature is not "none".
     // If the field is missing or "", the default value depends on the 'status'
     // field.  See the comment above.
@@ -1034,6 +1034,11 @@
       status: "stable",
       base_feature: "CssPaintingForSpellingGrammarErrors",
     },
+    {
+      // Ignore the stylesheet encoding when parsing URLs, always using UTF-8.
+      // See crbug.com/1485525.
+      name: "CSSParserIgnoreCharsetForURLs",
+    },
     {
       // crbug.com/1443291: Phrase Line Breaking
       name: "CSSPhraseLineBreak",