Mac: Only clear the background of CoreAnimation plugins if we're going to draw into them.

webkit/glue/plugins/webplugin_delegate_impl_mac.mm

Index: webkit/glue/plugins/webplugin_delegate_impl_mac.mm
diff --git a/webkit/glue/plugins/webplugin_delegate_impl_mac.mm b/webkit/glue/plugins/webplugin_delegate_impl_mac.mm
index cca8695563ab957e1abb598f7e652149fd331201..33747d99954c5cfeb7c61ae1db7868cd7708a126 100644
--- a/webkit/glue/plugins/webplugin_delegate_impl_mac.mm
+++ b/webkit/glue/plugins/webplugin_delegate_impl_mac.mm
@@ -981,14 +981,15 @@ void WebPluginDelegateImpl::DrawLayerInSurface() {
   if (!windowed_handle())
     return;
 
-  surface_->StartDrawing();
-
   [renderer_ beginFrameAtTime:CACurrentMediaTime() timeStamp:NULL];
   if (CGRectIsEmpty([renderer_ updateBounds])) {
     // If nothing has changed, we are done.
     [renderer_ endFrame];
     return;
   }
+
+  surface_->StartDrawing();
+
   CGRect layerRect = [layer_ bounds];
   [renderer_ addUpdateRect:layerRect];
   [renderer_ render];