Cleanup TouchEvent not to derive from MouseRelatedEvent

MouseReleatedEvent has coordinate fields that make no sense for a
TouchEvent (where the co-ordinates are in each TouchObject).  We should
derive from UIEventWithKeyState directly instead of using
MouseRelatedEvent.

There are no web-exposed changes here.  I've left the confusing
TouchEvent.initTouchEvent signature the same, ignoring the parameters
Safari has for screen/window co-ordinates (it looks like these would
never be used for anything before).

BUG=None

Review URL: https://codereview.chromium.org/293153003

git-svn-id: svn://svn.chromium.org/blink/trunk@174737 bbb929c8-8fbe-4397-9dbb-9b2b20218538