New UMA stats for stacking container opt-in measurements.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 6b944ddb48089a66743a8b96b4f128eea1a6ee38..548b5cf6f1748510fa0c1479696a618d5ebbbc80 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -1125,6 +1125,10 @@ protected:
     unsigned m_isUnclippedDescendant : 1;
 
     unsigned m_needsCompositedScrolling : 1;
+    unsigned m_needsCompositedScrollingHasBeenRecorded : 1;
+    unsigned m_willUseCompositedScrollingHasBeenRecorded : 1;
+
+    unsigned m_isScrollableAreaHasBeenRecorded : 1;
 
     // If this is true, then no non-descendant appears between any of our
     // descendants in stacking order. This is one of the requirements of being