Defer CGLFlushDrawable until OSX-requested drawRect to avoid spinning when window is obscured.

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 e22b0432aba31b7b1e5c1b63eaf8354d39f294cb..e742ce9967a77bf0d90cce5114b4d4b75a25e600 100644
--- a/content/browser/renderer_host/render_widget_host_impl.cc
+++ b/content/browser/renderer_host/render_widget_host_impl.cc
@@ -615,6 +615,10 @@ 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.