Prevent touch-based synthetic scroll gesture from causing a fling.

This patch introduces a waiting period at the end of touch-based scroll before
lifting the pointer. This ensures that the scroll doesn't cause an unwanted
fling when lifting the pointer.

The time to wait is provided by the SyntheticGestureTarget. Currently the same
hard-coded value is used on all platforms.

BUG=321114

Review URL: https://codereview.chromium.org/85933002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237573 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237573