Ensure Multi-finger tap generates both GestureScrollBegin and GestureScrollEnd.

There are two possible solutions to this bug:
1. Add a GestureScrollBegin.
2. Create a new state which allows scrolling but not tapping, and transition
   to that state on the release of a finger during multi-finger tap.

Option 1 is significantly simpler, and doesn't make the GR state machine any
more complicated. Option 2 prevents sending GestureScrollBegin and
GestureScrollEnd events which may not be accompanied by any GestureScrollUpdate
events.

I believe that option 1 is the correct choice here.

BUG=350121

Review URL: https://codereview.chromium.org/189583007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256004 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256004