Prepare CSSKeyframesMemoryReduction for finch experiment

This patch adds a Chromium feature for CSSKeyframesMemoryReduction and
enables it in field trial testing config. This is preparation for a
Canary/Dev finch experiment.

Bug: 1141814
Change-Id: I03c1c9beca18bdf4b286704f60b6895f60835d12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518097Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824639}

content/child/runtime_features.cc

@@ -338,6 +338,8 @@ void SetRuntimeFeaturesFromChromiumFeatures() {
           {"BlockHTMLParserOnStyleSheets",
            blink::features::kBlockHTMLParserOnStyleSheets},
           {"CSSColorSchemeUARendering", features::kCSSColorSchemeUARendering},
+          {"CSSKeyframesMemoryReduction",
+           blink::features::kCSSKeyframesMemoryReduction},
           {"CSSReducedFontLoadingLayoutInvalidations",
            blink::features::kCSSReducedFontLoadingLayoutInvalidations},
           {"CSSMatchedPropertiesCacheDependencies",

testing/variations/fieldtrial_testing_config.json

@@ -1235,6 +1235,27 @@
             ]
         }
     ],
+    "CSSKeyframesMemoryReduction": [
+        {
+            "platforms": [
+                "android",
+                "android_weblayer",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CSSKeyframesMemoryReduction"
+                    ]
+                }
+            ]
+        }
+    ],
     "CSSMatchedPropertiesCacheDependencies": [
         {
             "platforms": [

third_party/blink/common/features.cc

@@ -798,5 +798,12 @@ const base::Feature kWebRtcLibvpxEncodeNV12{"WebRtcLibvpxEncodeNV12",
 // the page is frozen.
 const base::Feature kLoadingTasksUnfreezable{"LoadingTasksUnfreezable",
                                              base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Changes ScopedStyleResolve and StyleEngine to use the already stored
+// StyleSheetCollection to find @keyframes rules instead of creating their
+// own hashmaps, so that we can save memory when there are web components
+// with @keyframes rules in their stylesheets.
+const base::Feature kCSSKeyframesMemoryReduction{
+    "CSSKeyframesMemoryReduction", base::FEATURE_DISABLED_BY_DEFAULT};
 }  // namespace features
 }  // namespace blink

third_party/blink/public/common/features.h

@@ -327,6 +327,8 @@ BLINK_COMMON_EXPORT extern const base::Feature kWebAppEnableProtocolHandlers;
 BLINK_COMMON_EXPORT extern const base::Feature kWebRtcLibvpxEncodeNV12;
 
 BLINK_COMMON_EXPORT extern const base::Feature kLoadingTasksUnfreezable;
+
+BLINK_COMMON_EXPORT extern const base::Feature kCSSKeyframesMemoryReduction;
 }  // namespace features
 }  // namespace blink