Mus: Avoid deadlock during teardown

services/ui/gpu/gpu_main.h

Index: services/ui/gpu/gpu_main.h
diff --git a/services/ui/gpu/gpu_main.h b/services/ui/gpu/gpu_main.h
index ede99136a4766a7cfe7ed398efa29e26ec880037..31e203e1cd0d4952315b8759ce143b08234791f3 100644
--- a/services/ui/gpu/gpu_main.h
+++ b/services/ui/gpu/gpu_main.h
@@ -29,6 +29,7 @@ class GpuMain : public gpu::GpuSandboxHelper {
 
  private:
   void InitOnGpuThread();
+  void TearDownOnCompositorThread();
   void TearDownOnGpuThread();
   void CreateOnGpuThread(mojom::GpuServiceInternalRequest request);
 
@@ -47,6 +48,9 @@ class GpuMain : public gpu::GpuSandboxHelper {
   // The thread that handles IO events for GpuService.
   base::Thread io_thread_;
 
+  // The thread used for the display compositor.
+  base::Thread compositor_thread_;
+
   base::WeakPtrFactory<GpuMain> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(GpuMain);