VCD: Fix a DCHECK failure when destroying CameraHalDispatcherImpl

I think |camera_hal_server_callbacks_| which is a mojo::Receiver, should
be reset on the proxy thread just like what we did for |receiver_set_|.

Bug: b/175662399
Test: Manually
Change-Id: If639add3be125f6c5b0ba36adcea0cb285987a8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2592536
Auto-Submit: Wei Lee <wtlee@chromium.org>
Commit-Queue: Shik Chen <shik@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837019}

media/capture/video/chromeos/camera_hal_dispatcher_impl.cc

@@ -657,6 +657,7 @@ void CameraHalDispatcherImpl::StopOnProxyThread() {
   // Close |cancel_pipe_| to quit the loop in WaitForIncomingConnection.
   cancel_pipe_.reset();
   client_observers_.clear();
+  camera_hal_server_callbacks_.reset();
   camera_hal_server_.reset();
   receiver_set_.Clear();
 }