Add method to reset the timestamp on a gesture and call it when transferring a gesture from a mouse down to a mouse up event.

BUG=265599
TEST=N/A

Review URL: https://chromiumcodereview.appspot.com/22430011

git-svn-id: svn://svn.chromium.org/blink/trunk@155872 bbb929c8-8fbe-4397-9dbb-9b2b20218538