Disable flaky OverviewWidgetStackingOrder

TBR=sammiequon@chromium.org

Bug: 822049
Change-Id: I25c7cf6bd7d4f1641c37afaaa176d7b24f674c39
Reviewed-on: https://chromium-review.googlesource.com/963153Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543256}

ash/wm/overview/window_selector_unittest.cc

@@ -2591,8 +2591,9 @@ TEST_F(WindowSelectorTest, WindowItemTitleCloseVisibilityOnDrag) {
   EXPECT_EQ(1.f, item2->GetCloseButtonOpacityForTesting());
 }
 
+// http://crbug.com/822049
 // Tests that overview widgets are stacked in the correct order.
-TEST_F(WindowSelectorTest, OverviewWidgetStackingOrder) {
+TEST_F(WindowSelectorTest, DISABLED_OverviewWidgetStackingOrder) {
   // Helper function to get the index of |child|, give its parent window
   // |parent|. Given the same |parent|, the children with higher index will be
   // stacked above (but not neccessarily directly) the children with lower