Use full mapLocalToAncestor machinery in ScrollingCoordinator.

third_party/WebKit/Source/core/frame/VisualViewport.cpp

Index: third_party/WebKit/Source/core/frame/VisualViewport.cpp
diff --git a/third_party/WebKit/Source/core/frame/VisualViewport.cpp b/third_party/WebKit/Source/core/frame/VisualViewport.cpp
index 7c906789930d63e51e4c8119d057a6f27e402cda..5585ea1d3d4f0a34b20b0eacda600d6e4948f973 100644
--- a/third_party/WebKit/Source/core/frame/VisualViewport.cpp
+++ b/third_party/WebKit/Source/core/frame/VisualViewport.cpp
@@ -593,10 +593,10 @@ float VisualViewport::browserControlsAdjustment() const {
 }
 
 IntRect VisualViewport::scrollableAreaBoundingBox() const {
-  // This method should return the bounding box in the parent view's coordinate
-  // space; however, VisualViewport technically isn't a child of any Frames.
-  // Nonetheless, the VisualViewport always occupies the entire main frame so
-  // just return that.
+  // This method should return the bounding box in the top-level FrameView's
+  // coordinate space; however, VisualViewport technically isn't a child of any
+  // Frames.  Nonetheless, the VisualViewport always occupies the entire main
+  // frame so just return that.
   LocalFrame* frame = mainFrame();
 
   if (!frame || !frame->view())