use enum to specify deviceSource for fling animation

patch 3 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/304823002

git-svn-id: svn://svn.chromium.org/blink/trunk@175506 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Split image-currentsrc test into 3 separate tests

Following flakiness resulting from race conditions, I'm splitting the image-currentsrc
test into 3 separate layout tests.

BUG=380664

Review URL: https://codereview.chromium.org/317723003

git-svn-id: svn://svn.chromium.org/blink/trunk@175507 bbb929c8-8fbe-4397-9dbb-9b2b20218538