Determine right frame before sending context menu on two finger tap.

Changes:
* Refactor subframe hit testing out into handleGestureEvent()
* Only let innermost frame handle LongPress/LongTap/TwoFingerTap gesture events.

BUG=306244

Review URL: https://codereview.chromium.org/53643002

git-svn-id: svn://svn.chromium.org/blink/trunk@161463 bbb929c8-8fbe-4397-9dbb-9b2b20218538