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,

[dcheng, 2017/02/23 19:36:58]

Nit: Error => Status { SUCCESS, TRY_AGAIN };

?

[mthiesse, 2017/02/23 20:39:58]

Fun fact, I can't use TRY_AGAIN as an enum value, it seems there's a collision
with some Android headers somehow? I've changed it to RETRY.

Compile error:
gen/device/vr/vr_service.mojom-shared.h:137:3: error: expected identifier
  TRY_AGAIN,
  ^
../../third_party/android_tools/ndk/platforms/android-16/arch-arm/usr/include/netdb.h:132:19:
note: expanded from macro 'TRY_AGAIN'
#define TRY_AGAIN       2 /* Non-Authoritative Host not found, or SERVERFAIL */
                        ^
In file included from
../../content/browser/frame_host/render_frame_host_impl.cc:129:
In file included from ../../device/vr/vr_service_impl.h:12:
In file included from ../../device/vr/vr_device.h:11:
In file included from gen/device/vr/vr_service.mojom.h:32:
gen/device/vr/vr_service.mojom-shared.h:144:35: error: expected unqualified-id
    case VRVSyncProvider_Statuss::TRY_AGAIN:
                                  ^
../../third_party/android_tools/ndk/platforms/android-16/arch-arm/usr/include/netdb.h:132:19:
note: expanded from macro 'TRY_AGAIN'
#define TRY_AGAIN       2 /* Non-Authoritative Host not found, or SERVERFAIL */

+    ERROR_TRY_AGAIN,
+    ERROR_BAD_REQUEST,

[dcheng, 2017/02/23 19:36:58]

I would just omit BAD_REQUEST from the enum completely.

[mthiesse, 2017/02/23 19:41:35]

How would you recommend we handle the bad request case, where we call
mojo::ReportBadMessage, but still have to run the callback to prevent the
browser crashing?

[dcheng, 2017/02/23 19:45:27]

We shouldn't have to invoke the callback on a bad message. If we do, that seems
like a bug in Mojo itself.

(I'm following up with some people about the crash report in the bug. The crash
is in DumpWithoutCrashing, which shouldn't crash, but it's showing up as a
renderer kill...)

[mthiesse, 2017/02/23 20:39:58]

Acknowledged.

+  };
+
   // 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);

[dcheng, 2017/02/23 19:36:58]

Nit: 80 chars

[mthiesse, 2017/02/23 20:39:58]

Done.

 };
 
 interface VRDisplayClient {