Bridge content and blink for an experiment

For the experiment TurnOff2DAndOpacityCompositorAnimations, we have the
experimental feature setup on the content and blink side, but we have
not yet bridge these two sides, which requires to add 2 lines of code
in content/child/runtime_features.cc. This CL does the bridging.

Bug: 754471
Change-Id: I84b85fadb68a2cebc0f642b3814fe9448bdfe3f3
Reviewed-on: https://chromium-review.googlesource.com/1002882Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549203}

content/child/runtime_features.cc

@@ -330,6 +330,10 @@ void SetRuntimeFeaturesDefaultsAndUpdateFromArgs(
                                                 false);
   }
 
+  if (base::FeatureList::IsEnabled(
+          features::kTurnOff2DAndOpacityCompositorAnimations))
+    WebRuntimeFeatures::EnableTurnOff2DAndOpacityCompositorAnimations(true);
+
   if (base::FeatureList::IsEnabled(features::kRasterInducingScroll))
     WebRuntimeFeatures::EnableRasterInducingScroll(true);