Bug 597676. If there's no rootPresContext, we should never detect DOM modification. Makes printing paint again. r=dbaron a=blocking2.0:beta7

layout/base/FrameLayerBuilder.cpp

@@ -1696,7 +1696,7 @@ FrameLayerBuilder::DrawThebesLayer(ThebesLayer* aLayer,
 PRBool
 FrameLayerBuilder::CheckDOMModified()
 {
-  if (mRootPresContext &&
+  if (!mRootPresContext ||
       mInitialDOMGeneration == mRootPresContext->GetDOMGeneration())
     return PR_FALSE;
   if (mDetectedDOMModification) {