WebVR: Defer GetVSync calls until the current frame is submitted.

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 1049 matching lines @@
     },
     {
       name: "WebVR",
       origin_trial_feature_name: "WebVR1.1",
       status: "test",
     },
     {
       name: "WebVRExperimentalRendering",
       // Don't mark this as depends_on: ["WebVR"], we want to be able to turn
       // it on for just origin trial sites where the dependency is unset.
-      status: "experimental",
+      // We also need it for tests since the default mode requires re-entrant
+      // mojo event handling, and the Javascript mocks don't support that.
+      status: "test",
     },
     {
       name: "WebVTTRegions",
       status: "experimental",
     },
     {
       name: "Worklet",
       status: "experimental",
     },
     {
       name: "XSLT",
       status: "stable",
     },
   ],
 }