[iOS] [Media] Prevent page zoom if it started in the video element
https://bugs.webkit.org/show_bug.cgi?id=134640
<rdar://problem/16630794>

Reviewed by Sam Weinig.

In r170572 I attempted to prevent the page zoom from happening
when the user was attempting to fullscreen the video. That fix was
not complete, because it relied on the touchstart event being
sent before a gesture event. It turns out this doesn't always happen,
so we need to apply the same check in the gesture event - if the
gesture began with two fingers on the media, don't do a page zoom.
The exception is zooming out, if you've managed to make the video
cover the entire screen (also, the pinch-out-of-fullscreen gesture
does not get handled by WebKit).

* Modules/mediacontrols/mediaControlsiOS.js:
(ControllerIOS.prototype.handleBaseGestureChange): Add a check to
make sure our pinch didn't start in the video element.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@170815 268f45cc-cd09-0410-ab3c-d52691b4dbfc