Fix the test flakiness DragWindowFromShelfControllerTest.FlingWithHiddenHotseat

Bug: 1023162
Change-Id: Ia8d07cf6ea1742c27b781d5214e47b347e7d3317
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906458Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714325}

ash/home_screen/drag_window_from_shelf_controller_unittest.cc

@@ -35,6 +35,13 @@ class DragWindowFromShelfControllerTest : public AshTestBase {
     base::RunLoop().RunUntilIdle();
   }
 
+  void TearDown() override {
+    // Destroy |window_drag_controller_| so that its scheduled task won't get
+    // run after the test environment is gone.
+    window_drag_controller_.reset();
+    AshTestBase::TearDown();
+  }
+
   void StartDrag(aura::Window* window,
                  const gfx::Point& location_in_screen,
                  HotseatState hotseat_state) {