viz: Convert a sync api in ServerGpuMemoryBufferManager into async.

gpu/command_buffer/client/gpu_memory_buffer_manager.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef GPU_COMMAND_BUFFER_CLIENT_GPU_MEMORY_BUFFER_MANAGER_H_
 #define GPU_COMMAND_BUFFER_CLIENT_GPU_MEMORY_BUFFER_MANAGER_H_
 
 #include <memory>
 
 #include "gpu/gpu_export.h"
 #include "gpu/ipc/common/surface_handle.h"
 #include "ui/gfx/geometry/size.h"
 #include "ui/gfx/gpu_memory_buffer.h"
 
 namespace gpu {
 
 struct SyncToken;
 
 class GPU_EXPORT GpuMemoryBufferManager {
  public:
   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;
 };
 
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_CLIENT_GPU_MEMORY_BUFFER_MANAGER_H_