Bug 647462 - Video layer should not be ARGB by default; r=chris.double

layout/generic/nsVideoFrame.cpp

@@ -276,6 +276,7 @@ nsVideoFrame::BuildLayer(nsDisplayListBuilder* aBuilder,
 
   layer->SetContainer(container);
   layer->SetFilter(nsLayoutUtils::GetGraphicsFilterForFrame(this));
+  layer->SetContentFlags(Layer::CONTENT_OPAQUE);
   // Set a transform on the layer to draw the video in the right place
   gfxMatrix transform;
   transform.Translate(r.pos);