Refactor gpu::Buffer to allow different types of backing

ppapi/proxy/ppb_graphics_3d_proxy.cc

Index: ppapi/proxy/ppb_graphics_3d_proxy.cc
diff --git a/ppapi/proxy/ppb_graphics_3d_proxy.cc b/ppapi/proxy/ppb_graphics_3d_proxy.cc
index 20c55de7b9acb09cda5016fda16ddfcd672acc7b..85692d40569bd2273f1a140461bdf993a78bf26c 100644
--- a/ppapi/proxy/ppb_graphics_3d_proxy.cc
+++ b/ppapi/proxy/ppb_graphics_3d_proxy.cc
@@ -297,9 +297,11 @@ void PPB_Graphics3D_Proxy::OnMsgCreateTransferBuffer(
         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;