Add a feature for not compositing small scrollers

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5

 {
   // http://dev.chromium.org/blink/runtime-enabled-features
   //
   // This list is used to generate RuntimeEnabledFeatures.h/cpp which contains
   // a class that stores static enablers for all experimental features.
 
   parameters: {
     // Each feature can be assigned a "status":
     // status=stable -> Enable this in all Blink configurations.  We are committed to these APIs indefinitely.
     // status=experimental -> In-progress features, Web Developers might play with, but are not on by default in stable.
@@ skipping 674 matching lines @@
     },
     {
       name: "NetworkInformation",
       status: "stable",
     },
     // Not a web exposed feature, enabled from the command line.
     {
       name: "NewRemotePlaybackPipeline",
     },
     {
+      name: "NotCompositeSmallScroller",

[flackr, 2017/06/05 17:50:41]

Suggestion, how about SkipCompositingSmallScrollers or
CompositeOnlyLargeScrollers?

[yigu, 2017/06/06 20:01:10]

Done.

+      status: "experimental",

[flackr, 2017/06/05 17:50:41]

I think status experimental means this would be enabled with
--experimental-web-platform-features which I don't think we should do until this
feature is implemented.

[yigu, 2017/06/06 20:01:10]

Have updated it to "test".

+    },
+    {
       name: "NotificationBadge",
       status: "stable",
     },
     {
       name: "NotificationConstructor",
       status: "stable",
     },
     // NotificationContentImage is not available in all platforms
     {
       name: "NotificationContentImage",
@@ skipping 410 matching lines @@
     {
       name: "Worklet",
       status: "experimental",
     },
     {
       name: "XSLT",
       status: "stable",
     },
   ],
 }