Scroll bars continue to scroll when finger leaves the scroll bar.

This patch also ensures that the thumb is deactivated when the finger
is released. Previously this didn't occur on fling start.

Scroll bars become active on tap down instead of scroll begin.

This disables fling for scroll bars.

BUG=324479
TEST=fast/events/touch/gesture/gesture-scrollbar.html, 
fast/events/touch/gesture/gesture-scrollbar-fling.html

Review URL: https://codereview.chromium.org/99013004

git-svn-id: svn://svn.chromium.org/blink/trunk@163298 bbb929c8-8fbe-4397-9dbb-9b2b20218538