Prevent browser crash resulting from misbehaving WebVR renderer requesting multiple VSyncs.

device/vr/vr_service.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module device.mojom;
 
 import "mojo/common/time.mojom";
 
 // A field of view, given by 4 degrees describing the view from a center point.
 struct VRFieldOfView {
@@ skipping 79 matching lines @@
 
   RequestPresent(bool secureOrigin) => (bool success);
   ExitPresent();
   SubmitFrame(VRPose? pose);
   UpdateLayerBounds(int16 frameId, VRLayerBounds leftBounds,
                     VRLayerBounds rightBounds);
   GetVRVSyncProvider(VRVSyncProvider& request);
 };
 
 interface VRVSyncProvider {
+  enum Error {
+    ERROR_NONE,
+    ERROR_TRY_AGAIN,
+    ERROR_BAD_REQUEST,
+  };
+
   // The frameId maps a VSync to a frame arriving from the compositor. IDs will
   // be reused after the frame arrives from the compositor. Negative IDs imply
   // no mapping.
-  GetVSync() => (VRPose? pose, mojo.common.mojom.TimeDelta time, int16 frameId);
+  GetVSync() => (VRPose? pose, mojo.common.mojom.TimeDelta time, int16 frameId, Error error);
 };
 
 interface VRDisplayClient {
   OnChanged(VRDisplayInfo display);
   OnExitPresent();
   OnBlur();
   OnFocus();
   OnActivate(VRDisplayEventReason reason);
   OnDeactivate(VRDisplayEventReason reason);
 };