viz: Convert a sync api in ServerGpuMemoryBufferManager into async.

gpu/command_buffer/client/gpu_memory_buffer_manager.h

Index: gpu/command_buffer/client/gpu_memory_buffer_manager.h
diff --git a/gpu/command_buffer/client/gpu_memory_buffer_manager.h b/gpu/command_buffer/client/gpu_memory_buffer_manager.h
index b75780bbc14921cd33d7646272734e456ff1861f..de0f4e71734700e08683d2f74c3d589291ab0e00 100644
--- a/gpu/command_buffer/client/gpu_memory_buffer_manager.h
+++ b/gpu/command_buffer/client/gpu_memory_buffer_manager.h
@@ -21,14 +21,16 @@ class GPU_EXPORT GpuMemoryBufferManager {
   GpuMemoryBufferManager();
   virtual ~GpuMemoryBufferManager();
 
-  // Creates a GpuMemoryBuffer that can be shared with another process.
+  // Creates a GpuMemoryBuffer that can be shared with another process. It can
+  // be called on any thread.
   virtual std::unique_ptr<gfx::GpuMemoryBuffer> CreateGpuMemoryBuffer(
       const gfx::Size& size,
       gfx::BufferFormat format,
       gfx::BufferUsage usage,
       gpu::SurfaceHandle surface_handle) = 0;
 
-  // Associates destruction sync point with |buffer|.
+  // Associates destruction sync point with |buffer|. It can be called on any
+  // thread.
   virtual void SetDestructionSyncToken(gfx::GpuMemoryBuffer* buffer,
                                        const gpu::SyncToken& sync_token) = 0;
 };