Refactor gpu::Buffer to allow different types of backing

ppapi/proxy/ppapi_command_buffer_proxy.cc

Index: ppapi/proxy/ppapi_command_buffer_proxy.cc
diff --git a/ppapi/proxy/ppapi_command_buffer_proxy.cc b/ppapi/proxy/ppapi_command_buffer_proxy.cc
index a1ac34748feeb061abeb76ee151bdebf4c0ce576..1e4820d5ec951a50e0822c8cb496989b178c58af 100644
--- a/ppapi/proxy/ppapi_command_buffer_proxy.cc
+++ b/ppapi/proxy/ppapi_command_buffer_proxy.cc
@@ -136,9 +136,7 @@ scoped_refptr<gpu::Buffer> PpapiCommandBufferProxy::CreateTransferBuffer(
     }
   }
 
-  scoped_refptr<gpu::Buffer> buffer =
-      new gpu::Buffer(shared_memory.Pass(), handle.size());
-  return buffer;
+  return gpu::MakeBufferFromSharedMemory(shared_memory.Pass(), handle.size());
 }
 
 void PpapiCommandBufferProxy::DestroyTransferBuffer(int32 id) {