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 a404cae09d9c5144c1504949a3e72824ee821c23..0932f87567ca93ceb01d3210181ca0e595a1b1bf 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -50,6 +50,7 @@
 #include "core/rendering/RenderLayerScrollableArea.h"
 
 #include "wtf/OwnPtr.h"
+#include "wtf/TriState.h"
 
 #include "core/rendering/RenderLayerFilterInfo.h"
 
@@ -1143,7 +1144,7 @@ protected:
 
     unsigned m_isUnclippedDescendant : 1;
 
-    unsigned m_needsCompositedScrolling : 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
@@ -1184,6 +1185,8 @@ protected:
 
     unsigned m_hasFilterInfo : 1;
 
+    TriState m_needsCompositedScrolling;
+
     BlendMode m_blendMode;
 
     RenderLayerModelObject* m_renderer;