Bug 1158546 - Remove nsDisplayHeaderFooter::mFrame in favor of nsDisplayItem::mFrame; r=roc

There is no need to have two separate variables for the same thing
in the same object.

layout/generic/nsPageFrame.cpp

@@ -40,6 +40,10 @@ NS_NewPageFrame(nsIPresShell* aPresShell, nsStyleContext* aContext)
 
 NS_IMPL_FRAMEARENA_HELPERS(nsPageFrame)
 
+NS_QUERYFRAME_HEAD(nsPageFrame)
+  NS_QUERYFRAME_ENTRY(nsPageFrame)
+NS_QUERYFRAME_TAIL_INHERITING(nsContainerFrame)
+
 nsPageFrame::nsPageFrame(nsStyleContext* aContext)
 : nsContainerFrame(aContext)
 {
@@ -480,7 +484,7 @@ static gfx::Matrix4x4 ComputePageTransform(nsIFrame* aFrame, float aAppUnitsPerP
 class nsDisplayHeaderFooter : public nsDisplayItem {
 public:
   nsDisplayHeaderFooter(nsDisplayListBuilder* aBuilder, nsPageFrame *aFrame)
-    : nsDisplayItem(aBuilder, aFrame), mFrame(aFrame)
+    : nsDisplayItem(aBuilder, aFrame)
     , mDisableSubpixelAA(false)
   {
     MOZ_COUNT_CTOR(nsDisplayHeaderFooter);
@@ -493,7 +497,12 @@ public:
 
   virtual void Paint(nsDisplayListBuilder* aBuilder,
                      nsRenderingContext* aCtx) override {
-    mFrame->PaintHeaderFooter(*aCtx, ToReferenceFrame(), mDisableSubpixelAA);
+#ifdef DEBUG
+    nsPageFrame* pageFrame = do_QueryFrame(mFrame);
+    MOZ_ASSERT(pageFrame, "We should have an nsPageFrame");
+#endif
+    static_cast<nsPageFrame*>(mFrame)->
+      PaintHeaderFooter(*aCtx, ToReferenceFrame(), mDisableSubpixelAA);
   }
   NS_DISPLAY_DECL_NAME("HeaderFooter", nsDisplayItem::TYPE_HEADER_FOOTER)
 
@@ -506,7 +515,6 @@ public:
     mDisableSubpixelAA = true;
   }
 protected:
-  nsPageFrame* mFrame;
   bool mDisableSubpixelAA;
 };
 

layout/generic/nsPageFrame.h

@@ -16,6 +16,8 @@ class nsSharedPageData;
 class nsPageFrame final : public nsContainerFrame {
 
 public:
+  NS_DECL_QUERYFRAME_TARGET(nsPageFrame)
+  NS_DECL_QUERYFRAME
   NS_DECL_FRAMEARENA_HELPERS
 
   friend nsPageFrame* NS_NewPageFrame(nsIPresShell* aPresShell,