Delete background tab IOSurfaces on Mac.

content/common/view_messages.h

Index: content/common/view_messages.h
diff --git a/content/common/view_messages.h b/content/common/view_messages.h
index 7a5433958531b5b364fb2cb3391b779d60f6cd59..396331629cea679dc40d365148901f531104c4fa 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -499,7 +499,8 @@ IPC_STRUCT_BEGIN(ViewHostMsg_UpdateRect_Params)
   // The size of the RenderView when this message was generated.  This is
   // included so the host knows how large the view is from the perspective of
   // the renderer process.  This is necessary in case a resize operation is in
-  // progress.
+  // progress. If auto-resize is enabled, this should update the corresponding
+  // view size.
   IPC_STRUCT_MEMBER(gfx::Size, view_size)
 
   // New window locations for plugin child windows.
@@ -1910,6 +1911,15 @@ IPC_MESSAGE_ROUTED2(ViewHostMsg_AcceleratedSurfaceBuffersSwapped,
                     uint64 /* surface_handle */)
 #endif
 
+// This message is synthesized by GpuProcessHost to pass through a swap message
+// to the RenderWidgetHelper. This allows GetBackingStore to block for either a
+// software or GPU frame.
+IPC_MESSAGE_ROUTED4(ViewHostMsg_CompositorSurfaceBuffersSwapped,
+                    int32 /* surface id */,
+                    uint64 /* surface_handle */,
+                    int32 /* route_id */,
+                    int32 /* gpu_process_host_id */)
+
 // Opens a file asynchronously. The response returns a file descriptor
 // and an error code from base/platform_file.h.
 IPC_MESSAGE_ROUTED3(ViewHostMsg_AsyncOpenFile,