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

third_party/WebKit/LayoutTests/vr/resources/mock-vr-service.js

 'use strict';
 
 let mockVRService = loadMojoModules(
     'mockVRService',
     ['mojo/public/js/bindings',
      'device/vr/vr_service.mojom',
     ]).then(mojo => {
   let [bindings, vr_service] = mojo.modules;
 
   class MockVRDisplay {
     constructor(interfaceProvider, displayInfo, service) {
       this.bindingSet_ = new bindings.BindingSet(vr_service.VRDisplay);
       this.displayClient_ = new vr_service.VRDisplayClientPtr();
       this.displayInfo_ = displayInfo;
       this.service_ = service;
       this.vsync_provider_ = new MockVRVSyncProvider();
 
       interfaceProvider.addInterfaceOverrideForTesting(
           vr_service.VRDisplay.name,
           handle => this.bindingSet_.addBinding(this, handle));
 
       if (service.client_) {
         this.notifyClientOfDisplay();
       }
     }
 
-    requestPresent(secureOrigin) {
+    requestPresent(secureOrigin, submitFrameClient) {
+      this.submitFrameClient_ = submitFrameClient;
       return Promise.resolve({success: true});
     }
 
+    submitFrame(frameId, mailboxHolder) {
+      // Trigger the submit completion callbacks here. WARNING: The
+      // Javascript-based mojo mocks are *not* re-entrant.  In the current
+      // default implementation, Javascript calls display.submitFrame, and the
+      // corresponding C++ code uses a reentrant mojo call that waits for
+      // onSubmitFrameTransferred to indicate completion. This never finishes
+      // when using the mocks since the incoming calls are queued until the
+      // current execution context finishes. As a workaround, use the alternate
+      // "WebVRExperimentalRendering" mode which works without reentrant calls,
+      // the code only checks for completion on the *next* frame, see the
+      // corresponding option setting in RuntimeEnabledFeatures.json5.
+      this.submitFrameClient_.onSubmitFrameTransferred();
+      this.submitFrameClient_.onSubmitFrameRendered();
+    }
+
     setPose(pose) {
       if (pose == null) {
         this.vsync_provider_.pose_ = null;
       } else {
         this.vsync_provider_.initPose();
         this.vsync_provider_.fillPose(pose);
       }
     }
 
-    getVRVSyncProvider(request) { this.vsync_provider_.bind(request); }
+    getVRVSyncProvider(request) {
+      this.vsync_provider_.bind(request);
+    }
 
     forceActivate(reason) {
       this.displayClient_.onActivate(reason);
     }
 
     notifyClientOfDisplay() {
       let displayPtr = new vr_service.VRDisplayPtr();
       let request = bindings.makeRequest(displayPtr);
       let binding = new bindings.Binding(
           vr_service.VRDisplay,
@@ skipping 95 matching lines @@
   return new MockVRService(mojo.frameInterfaces);
 });
 
 function vr_test(func, vrDisplays, name, properties) {
   mockVRService.then( (service) => {
     service.setVRDisplays(vrDisplays);
     let t = async_test(name, properties);
     func(t, service);
   });
 }