Bug 848421. Part 1. Handle having no frame for the body element when determining the image filter to use for drawing the page background. r=roc

Just use nsCSSRendering::FindBackground like we do everywhere else for finding the background, it handles this case correctly. Although this function is used for other images besides backgrounds, when it is called on a canvas frame it can only be for a background image.

layout/base/nsLayoutUtils.cpp

@@ -3771,10 +3771,14 @@ GraphicsFilter
 nsLayoutUtils::GetGraphicsFilterForFrame(nsIFrame* aForFrame)
 {
   GraphicsFilter defaultFilter = gfxPattern::FILTER_GOOD;
-  nsIFrame *frame = nsCSSRendering::IsCanvasFrame(aForFrame) ?
-    nsCSSRendering::FindBackgroundStyleFrame(aForFrame) : aForFrame;
+  nsStyleContext *sc;
+  if (nsCSSRendering::IsCanvasFrame(aForFrame)) {
+    nsCSSRendering::FindBackground(aForFrame->PresContext(), aForFrame, &sc);
+  } else {
+    sc = aForFrame->StyleContext();
+  }
 
-  switch (frame->StyleSVG()->mImageRendering) {
+  switch (sc->StyleSVG()->mImageRendering) {
   case NS_STYLE_IMAGE_RENDERING_OPTIMIZESPEED:
     return gfxPattern::FILTER_FAST;
   case NS_STYLE_IMAGE_RENDERING_OPTIMIZEQUALITY: