Expose explicit fling cancellation hooks via WebView

Add a public method to WebView allowing cancellation of an active fling
animation.  This allows cancellation of flings without the need to
create synthetic GestureFlingCancel events.

Corresponding Chromium patch: https://codereview.chromium.org/49253002 

BUG=309649

Review URL: https://codereview.chromium.org/53093003

git-svn-id: svn://svn.chromium.org/blink/trunk@162220 bbb929c8-8fbe-4397-9dbb-9b2b20218538