cros: Fix accidental enlargement of app icon on two finger tap.

Our logic assumed that an ET_GESTURE_END will always come
after an ET_GESTURE_TAP_DOWN, this isn't true.

When a Two-finger tap (ET_GESTURE_TWO_FINGER_TAP) occurs the
event stream is ET_GESTURE_TAP_DOWN -> ET_GESTURE_TAP_CANCEL ->
ET_GESTURE_TWO_FINGER_TAP with no ET_GESTURE_END.

This CL fixes the issue by clearing the set state by ET_GESTURE_TAP_DOWN
on ET_GESTURE_TAP_CANCEL.

Bug: 809751
Change-Id: Iefeda869c70a75ba09a21fe4449b337a5cb7a826
Reviewed-on: https://chromium-review.googlesource.com/905864Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534888}

ash/shelf/shelf_button.cc

@@ -556,6 +556,7 @@ void ShelfButton::OnGestureEvent(ui::GestureEvent* event) {
           base::Bind(&ShelfButton::OnTouchDragTimer, base::Unretained(this)));
       event->SetHandled();
       break;
+    case ui::ET_GESTURE_TAP_CANCEL:
     case ui::ET_GESTURE_END:
       drag_timer_.Stop();
       ClearState(STATE_HOVERED);