Removing seemingly needless indirection in ScrollingCoordinator.

All these methods were really doing was calling the equivalents in
ScrollableArea. I removed the methods and adjusted the call sites to
call the ScrollableArea methods directly.

BUG=

Review URL: https://codereview.chromium.org/197473011

git-svn-id: svn://svn.chromium.org/blink/trunk@169184 bbb929c8-8fbe-4397-9dbb-9b2b20218538