Bug 1091283 - Ensure that pinch gesture events actually get processed. r=botond

Currently only pinch gesture events getting created in the GestureEventListener
code will make it to AsyncPanZoomController::HandleGestureEvent. Any pinch
gesture events that the widget code dispatches into the APZCTreeManager get
ignored, which shouldn't be happening. We don't have any production code that
exercises these code paths at the moment, but fixing it up also reduces the
APIs exposed into the testing code.
Bug 1091283 - Allow using the existing gtest input helpers on APZCTreeManager as well. r=botond

Thanks to the magic of templates we can reuse the code we have for dispatching
common gestures into the AsyncPanZoomController.