Bug 1277123. Only clear the containing block clip when entering an out of flow with an unknown clip. r=mattwoodrow

The content clip is still valid, as we are only descending into content descendants.

The clip for content descendants that the root scroll frame creates so content doesn't overflow (visually or for event handling) the main scrollbar can get cleared otherwise.

layout/generic/nsFrame.cpp

@@ -2807,7 +2807,7 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
     // then we won't have stored clip data for it. We can just clear the clip
     // instead since we know we won't render anything, and the inner out-of-flow
     // frame will setup the correct clip for itself.
-    clipState.Clear();
+    clipState.SetClipForContainingBlockDescendants(nullptr);
     clipState.SetScrollClipForContainingBlockDescendants(nullptr);
   }