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

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5

Index: third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5
diff --git a/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5 b/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5
index 245fe6c84131be07f561d7e0534637bfd35b8c9e..b6f44951df0a615e4d2beef5904112a16152b7ff 100644
--- a/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5
+++ b/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5
@@ -1071,7 +1071,9 @@
       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",