ScrollingCoordinator shouldn't be RefCounted

There's no reason for this class to be RefCounted. We can just use the simpler
OwnPtr.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/159113003

git-svn-id: svn://svn.chromium.org/blink/trunk@166867 bbb929c8-8fbe-4397-9dbb-9b2b20218538