Disable WorkspaceLayoutManagerBackdropTest.OpenAppListInOverviewMode

The test WorkspaceLayoutManagerBackdropTest.OpenAppListInOverviewMode is flaking
so I'm disabling.

TBR= xiyuan@chromium.org
TEST=WorkspaceLayoutManagerBackdropTest.OpenAppListInOverviewMode

Bug: 838756
Change-Id: I68f90b668996a885ef37e047d453e6ecca12217c
Reviewed-on: https://chromium-review.googlesource.com/1042350Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555739}

ash/wm/workspace/workspace_layout_manager_unittest.cc

@@ -1547,7 +1547,8 @@ TEST_F(WorkspaceLayoutManagerBackdropTest,
   GetAppListTestHelper()->Dismiss();
 }
 
-TEST_F(WorkspaceLayoutManagerBackdropTest, OpenAppListInOverviewMode) {
+// TODO(minch): This test has a high flake rate. https://crbug.com/838756
+TEST_F(WorkspaceLayoutManagerBackdropTest, DISABLED_OpenAppListInOverviewMode) {
   WorkspaceController* wc = ShellTestApi(Shell::Get()).workspace_controller();
   WorkspaceControllerTestApi test_helper(wc);