Use full mapLocalToAncestor machinery in ScrollingCoordinator.

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
index f5f6a86dd3463eda42ea8a71a86622a9c72d28ce..1748a506bd107ad96c08360982751d491e57dbc8 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
@@ -246,7 +246,7 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin {
   virtual bool scrollbarsCanBeActive() const = 0;
 
   // Returns the bounding box of this scrollable area, in the coordinate system
-  // of the enclosing scroll view.
+  // of the top-level FrameView.
   virtual IntRect scrollableAreaBoundingBox() const = 0;
 
   virtual bool scrollAnimatorEnabled() const { return false; }