[Aura] Stop using consistent velocity as scroll start heuristic

We now subtract the slop radius from the first scroll,
which makes scroll start feel smooth without the sometimes
faulty consistent velocity heuristic.

This also brings Aura more in line with Android's behaviour.

BUG=335349
TEST=GestureRecognizerTest.GestureEventScroll, GestureRecognizerTest.*Scroll*

Review URL: https://codereview.chromium.org/136383006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248024 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248024