bindings: Flip defalut settings of V8ContextSnapshot

We ran a Finch experiment to measure the impact to create V8 contexts
from snapshot, and we got a good result from it.
And we sent "Intent to ship"[1] and got no objections for a week.
Now it's time to enable it by default.

[1] https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/llHBIsHBb8M


Bug: 764576, 760875
Change-Id: I14d0658bcaf50e74b020b101ae3d221f28248ece
Reviewed-on: https://chromium-review.googlesource.com/903523Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534654}

content/child/runtime_features.cc

@@ -410,8 +410,8 @@ void SetRuntimeFeaturesDefaultsAndUpdateFromArgs(
     WebRuntimeFeatures::EnableFeatureFromString(feature, false);
   }
 
-  if (base::FeatureList::IsEnabled(features::kV8ContextSnapshot))
-    WebRuntimeFeatures::EnableV8ContextSnapshot(true);
+  WebRuntimeFeatures::EnableV8ContextSnapshot(
+      base::FeatureList::IsEnabled(features::kV8ContextSnapshot));
 
   if (base::FeatureList::IsEnabled(features::kStopInBackground))
     WebRuntimeFeatures::EnableStopInBackground(true);

content/public/common/content_features.cc

@@ -548,7 +548,7 @@ const base::Feature kV8BackgroundCompile{"V8BackgroundCompile",
 
 // Enables to use a snapshot file in creating V8 contexts.
 const base::Feature kV8ContextSnapshot{"V8ContextSnapshot",
-                                       base::FEATURE_DISABLED_BY_DEFAULT};
+                                       base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables future V8 VM features
 const base::Feature kV8VmFuture{"V8VmFuture",