Re-enable tap down gesture event handling.

BUG=302752
TEST=Existing layout tests, manual testing with chromium passing
either tap down or show press events.

Review URL: https://codereview.chromium.org/37483002

git-svn-id: svn://svn.chromium.org/blink/trunk@160446 bbb929c8-8fbe-4397-9dbb-9b2b20218538