Events ignoring ack disposition receive synthetic acks.

Logic to handle events which ignore ack disposition has moved from the
|GestureEventFilter| to the |ImmediateInputRouter|, and |WebInputEventTraits|.

This will allow us to make more events ignore their ack disposition, including
touch events.

Gesture event types which will eventually ignore their ack disposition include 
Tap, ScrollBegin, PinchBegin, PinchEnd, etc.

BUG=302852,275611
TEST=ImmediateInputRouterTest.EventsIgnoringAckDispositionDontWaitForAcks,
ImmediateInputRouterTest.EventsIgnoringAckDispositionStayInOrder

Review URL: https://codereview.chromium.org/44983003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233946 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233946