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

device/vr/vr_service.mojom

Index: device/vr/vr_service.mojom
diff --git a/device/vr/vr_service.mojom b/device/vr/vr_service.mojom
index 19717b8c5cffa81b52babb983ae5ed7b2221627c..b8c8fa238f3d0293cc1b694af82c34ee4391b98b 100644
--- a/device/vr/vr_service.mojom
+++ b/device/vr/vr_service.mojom
@@ -97,10 +97,16 @@ interface VRDisplay {
 };
 
 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 {