Bug 579663. Part 5. Make display zoom items create a container layer. r=roc

layout/base/nsDisplayList.cpp

@@ -1379,7 +1379,7 @@ nsDisplayWrapList* nsDisplayClip::WrapWithClone(nsDisplayListBuilder* aBuilder,
 
 nsDisplayZoom::nsDisplayZoom(nsIFrame* aFrame, nsDisplayList* aList,
                              PRInt32 aAPD, PRInt32 aParentAPD)
-    : nsDisplayWrapList(aFrame, aList), mAPD(aAPD), mParentAPD(aParentAPD) {
+    : nsDisplayOwnLayer(aFrame, aList), mAPD(aAPD), mParentAPD(aParentAPD) {
   MOZ_COUNT_CTOR(nsDisplayZoom);
 }
 

layout/base/nsDisplayList.h

@@ -1607,9 +1607,9 @@ private:
 
 /**
  * nsDisplayZoom is used for subdocuments that have a different full zoom than
- * their parent documents.
+ * their parent documents. This item creates a container layer.
  */
-class nsDisplayZoom : public nsDisplayWrapList {
+class nsDisplayZoom : public nsDisplayOwnLayer {
 public:
   /**
    * @param aFrame is the root frame of the subdocument.