In bug1456322, we use `IsEventOutsideDragThreshold()` to check whether the touchend event is from dragging or touching at one point. However, this method which is broken on Android always returns true, so we can't use this method to distinguish touch and drag. The reason is that `mGestureDownPoint` is never set during draging, so everytime the distance we calculate in `IsEventOutsideDragThreshold()` would always larger than the threshold. The `mGestureDownPoint` is set in `BeginTrackingDragGesture()` which is never called during dragging.

Why is BeginTrackingDragGesture not called? Is this Android widget level issue? Ah, looks like so. Touch dragging is implemented on Windows only.

We should set the 'mGestureDownPoint' correctly when receiving 'touchstart' so that we can distinguish whether the 'touchend' comes from dragging or simply touching by calling 'IsEventOutsideDragThreshold()'.

Pushed by alwu@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/7fbec497af48 set mGestureDownPoint when receiving 'touchstart' event. r=smaug