Improve OverviewGrid::Shutdown efficiency.

Removing the items from the container one by one is inefficient, and at
shutting down, this can be done by emptying the container directly.

Bug: 1112835
Change-Id: Ib7529963763b1e772b6ba330eda3a3a3e2f6d380
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340319Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Jun Liu <yjliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796153}

ash/wm/overview/overview_controller_unittest.cc

@@ -699,8 +699,6 @@ TEST_F(OverviewControllerTest, FrameThrottling) {
               OnThrottlingStarted(testing::UnorderedElementsAreArray(windows)));
   controller->StartOverview();
 
-  EXPECT_CALL(observer, OnThrottlingStarted(::testing::_))
-      .Times(::testing::AnyNumber());
   EXPECT_CALL(observer, OnThrottlingEnded());
   controller->EndOverview();
   frame_throttling_controller->RemoveObserver(&observer);

ash/wm/overview/overview_grid.cc

@@ -402,12 +402,7 @@ void OverviewGrid::Shutdown() {
         single_animation_in_clamshell, minimized_in_tablet);
   }
 
-  while (!window_list_.empty()) {
-    RemoveItem(window_list_.back().get(), /*item_destroying=*/false,
-               /*reposition=*/false);
-  }
-
-  // RemoveItem() uses `overview_session_`, so clear it at the end.
+  window_list_.clear();
   overview_session_ = nullptr;
 }