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

This was initially committed at
https://codereview.chromium.org/99013004/,
and reverted at
https://codereview.chromium.org/102663003.

The revert was due to some failing content_browsertests. These tests
relied on the previous broken behaviour of the scrollbar. This was fixed
at https://codereview.chromium.org/110403002/, so it should be safe to
reland this patch.

BUG=324479

Review URL: https://codereview.chromium.org/105973004

git-svn-id: svn://svn.chromium.org/blink/trunk@163706 bbb929c8-8fbe-4397-9dbb-9b2b20218538