gpu: Add async pixel transfer interface, stub and tests.

gpu/command_buffer/service/common_decoder.h

Index: gpu/command_buffer/service/common_decoder.h
diff --git a/gpu/command_buffer/service/common_decoder.h b/gpu/command_buffer/service/common_decoder.h
index 94dec16bb1c7acabb2183e324ec2e160a8247262..28483605da0ac70895d5e5b2f9a4e01d9145137f 100644
--- a/gpu/command_buffer/service/common_decoder.h
+++ b/gpu/command_buffer/service/common_decoder.h
@@ -10,6 +10,7 @@
 #include <string>
 #include "base/memory/linked_ptr.h"
 #include "base/memory/scoped_ptr.h"
+#include "gpu/command_buffer/common/buffer.h"
 #include "gpu/command_buffer/service/cmd_parser.h"
 #include "gpu/gpu_export.h"
 
@@ -129,6 +130,9 @@ class GPU_EXPORT CommonDecoder : NON_EXPORTED_BASE(public AsyncAPIInterface) {
     return static_cast<T>(GetAddressAndCheckSize(shm_id, offset, size));
   }
 
+  // Get the actual shared memory buffer.
+  Buffer GetSharedMemoryBuffer(unsigned int shm_id);
+
  protected:
   // Executes a common command.
   // Parameters: