Explicitly enable CompositingOptimizations for web tests

This CL (temporarily) explicitly enable CompositingOptimizations for
web tests.

This is to separate the change of web test expectations from
crrev.com/c/2375886 which will enable the feature for web tests by
adjusting the application order of blink feature status and chromium
feature status.

Bug: 1121839
Change-Id: I55ef951dd6a18731b33cf18496551cd9c993162e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380213
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802483}

content/child/runtime_features.cc

@@ -604,6 +604,7 @@ void SetCustomizedRuntimeFeaturesFromCombinedArgs(
   // TODO(rodneyding): Investigate more on proper treatments of
   // these features.
   if (enable_experimental_web_platform_features) {
+    WebRuntimeFeatures::EnableCompositingOptimizations(true);
     WebRuntimeFeatures::EnableNetInfoDownlinkMax(true);
     WebRuntimeFeatures::EnableSignedExchangePrefetchCacheForNavigations(true);
     WebRuntimeFeatures::EnableSignedExchangeSubresourcePrefetch(true);

third_party/blink/web_tests/VirtualTestSuites

@@ -111,8 +111,7 @@
               "compositing/filters"
               ],
     "args": ["--stable-release-mode",
-             "--disable-auto-wpt-origin-isolation",
-             "--enable-blink-features=CompositingOptimizations"]
+             "--disable-auto-wpt-origin-isolation"]
   },
   {
     "prefix": "feature-policy-permissions",

third_party/blink/web_tests/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt

@@ -11,6 +11,19 @@
       "position": [-100, -100],
       "bounds": [200, 200],
       "transform": 1
+    },
+    {
+      "name": "LayoutNGBlockFlow (positioned) DIV id='software-parent'",
+      "bounds": [100, 100],
+      "contentsOpaque": true,
+      "backgroundColor": "#008000"
+    },
+    {
+      "name": "LayoutNGBlockFlow (positioned) DIV id='software-child'",
+      "position": [100, 100],
+      "bounds": [50, 50],
+      "contentsOpaque": true,
+      "backgroundColor": "#0000FF"
     }
   ],
   "transforms": [

third_party/blink/web_tests/virtual/stable/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt

@@ -11,19 +11,6 @@
       "position": [-100, -100],
       "bounds": [200, 200],
       "transform": 1
-    },
-    {
-      "name": "LayoutNGBlockFlow (positioned) DIV id='software-parent'",
-      "bounds": [100, 100],
-      "contentsOpaque": true,
-      "backgroundColor": "#008000"
-    },
-    {
-      "name": "LayoutNGBlockFlow (positioned) DIV id='software-child'",
-      "position": [100, 100],
-      "bounds": [50, 50],
-      "contentsOpaque": true,
-      "backgroundColor": "#0000FF"
     }
   ],
   "transforms": [