Refactor gpu::Buffer to allow different types of backing

ppapi/proxy/ppb_graphics_3d_proxy.cc

 // Copyright (c) 2012 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.
 
 #include "ppapi/proxy/ppb_graphics_3d_proxy.h"
 
 #include "gpu/command_buffer/client/gles2_implementation.h"
 #include "gpu/command_buffer/common/command_buffer.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/proxy/enter_proxy.h"
@@ skipping 279 matching lines @@
     uint32 size,
     int32* id,
     ppapi::proxy::SerializedHandle* transfer_buffer) {
   transfer_buffer->set_null_shmem();
   EnterHostFromHostResource<PPB_Graphics3D_API> enter(context);
   if (enter.succeeded()) {
     scoped_refptr<gpu::Buffer> buffer =
         enter.object()->CreateTransferBuffer(size, id);
     if (!buffer)
       return;
-    DCHECK(buffer->shared_memory());
+    gpu::SharedMemoryBufferBacking* backing =
+        static_cast<gpu::SharedMemoryBufferBacking*>(buffer->backing());
+    DCHECK(backing && backing->shared_memory());
     transfer_buffer->set_shmem(
-        TransportSHMHandle(dispatcher(), buffer->shared_memory()),
+        TransportSHMHandle(dispatcher(), backing->shared_memory()),
         buffer->size());
   } else {
     *id = -1;
   }
 }
 
 void PPB_Graphics3D_Proxy::OnMsgDestroyTransferBuffer(
     const HostResource& context,
     int32 id) {
   EnterHostFromHostResource<PPB_Graphics3D_API> enter(context);
@@ skipping 30 matching lines @@
     int32_t result,
     const HostResource& context) {
   dispatcher()->Send(new PpapiMsg_PPBGraphics3D_SwapBuffersACK(
       API_ID_PPB_GRAPHICS_3D, context, result));
 }
 #endif  // !defined(OS_NACL)
 
 }  // namespace proxy
 }  // namespace ppapi