Add GuaranteePersistence to PushLayer in DrawTargetCapture. (bug 1447144, r=bas)

--HG--
extra : amend_source : 4492b72ad9e306aeefadddd970b0e997de386239

gfx/2d/DrawTargetCapture.cpp

@@ -290,6 +290,10 @@ DrawTargetCaptureImpl::PushLayer(bool aOpaque,
   mPushedLayers.push_back(layer);
   DrawTarget::SetPermitSubpixelAA(aOpaque);
 
+  if (aMask) {
+    aMask->GuaranteePersistance();
+  }
+
   AppendCommand(PushLayerCommand)(aOpaque,
                                   aOpacity,
                                   aMask,