Support offscreen contexts which own their backing surface

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 893 matching lines @@
       name: "WebUSB",
       origin_trial_feature_name: "WebUSB2",
       status: "experimental",
     },
     {
       name: "WebVR",
       origin_trial_feature_name: "WebVR",
       status: "test",
     },
     {
+      name: "WebVRExperimentalRendering",

[amp, 2017/02/15 17:44:03]

You can add a 'depends_on' section here to limit this being turned on only when
WebVR is turned on (see the 'depends_on' docs at the top of the file).

[klausw, 2017/02/15 19:47:22]

Declaring it as a dependency means that there's no way to turn on the
optimizations in "origin trial only" mode. I've added a code comment to explain.

+      status: "test",
+    },
+    {
       name: "WebVTTRegions",
       status: "experimental",
     },
     {
       name: "V8IdleTasks",
     },
     {
       name: "VideoFullscreenOrientationLock",
     },
     {
@@ skipping 80 matching lines @@
     {
       name: "UnclosedFormControlIsInvalid",
       status: "experimental",
     },
     {
       name: "RestrictCompleteURLCharacterSet",
       status: "experimental",
     },
   ],
 }