Remove code only used by Mac single threaded compositing.

content/browser/renderer_host/render_widget_host_impl.cc

Index: content/browser/renderer_host/render_widget_host_impl.cc
diff --git a/content/browser/renderer_host/render_widget_host_impl.cc b/content/browser/renderer_host/render_widget_host_impl.cc
index fa181508f38625628351beda035fb23d15e0df82..9f064f2ab87642184c0d689aee9d208e9c4efa9c 100644
--- a/content/browser/renderer_host/render_widget_host_impl.cc
+++ b/content/browser/renderer_host/render_widget_host_impl.cc
@@ -837,10 +837,6 @@ BackingStore* RenderWidgetHostImpl::GetBackingStore(bool force_create) {
   do {
     TRACE_EVENT0("renderer_host", "GetBackingStore::WaitForUpdate");
 
-#if defined(OS_MACOSX)
-    view_->AboutToWaitForBackingStoreMsg();
-#endif
-
     // When we have asked the RenderWidget to resize, and we are still waiting
     // on a response, block for a little while to see if we can't get a response
     // before returning the old (incorrectly sized) backing store.