Sticky keys modifies an entire scroll sequence instead of just a single event.

A scroll sequence starts with a fling cancel, followed by a series of scroll
events and ends with a fling start event. In the oneshot sticky keys case, we
now modify the entire sequence with the modifier key before disabling sticky
keys.

BUG=308662
TEST=updated unit tests

Review URL: https://codereview.chromium.org/81863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236834 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236834