overview: Fix crash after flinging item.

Some states were not reset after successful fling.

Test: manual
Bug: 850806
Change-Id: I826501e6921163b9f5eaa36dd7a1c6d5de940e94
Reviewed-on: https://chromium-review.googlesource.com/1092152Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565659}

ash/wm/overview/overview_window_drag_controller.cc

@@ -168,6 +168,9 @@ void OverviewWindowDragController::Fling(const gfx::Point& location_in_screen,
     if (std::abs(velocity_y) > kFlingToCloseVelocityThreshold) {
       item_->AnimateAndCloseWindow(
           (location_in_screen - initial_event_location_).y() < 0);
+      did_move_ = false;
+      item_ = nullptr;
+      current_drag_behavior_ = DragBehavior::kNoDrag;
       return;
     }
   }