use enum to specify deviceSource for fling animation

Patch 1 of 4 to specify the deviceSource of a desired fling animation
request via enum throughout Blink and Chromium.

BUG=343327

Review URL: https://codereview.chromium.org/307733002

git-svn-id: svn://svn.chromium.org/blink/trunk@175151 bbb929c8-8fbe-4397-9dbb-9b2b20218538