Disabling OverviewSessionTest.DropTargetOnCorrectDisplayForDraggingFromOverview

Test has been flaky in the recent runs:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=ash_unittests&tests=DropTargetOnCorrectDisplayForDraggingFromOverview

TBR=sammiequon@chromium.org

Change-Id: I30ff04ef28a9a4be996f76a7ae88c23d68b00771
Bug: 1008408
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1827501Reviewed-by: Sebastien Lalancette <seblalancette@chromium.org>
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Auto-Submit: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700295}

ash/wm/overview/overview_session_unittest.cc

@@ -1298,7 +1298,8 @@ TEST_F(OverviewSessionTest, DropTargetOnCorrectDisplayForDraggingFromTop) {
 
 // Tests that dragging a window from overview creates a drop target on the same
 // display.
-TEST_F(OverviewSessionTest, DropTargetOnCorrectDisplayForDraggingFromOverview) {
+// Disabled, test is flaky: crbug.com/1008408
+TEST_F(OverviewSessionTest, DISABLED_DropTargetOnCorrectDisplayForDraggingFromOverview) {
   UpdateDisplay("600x600,600x600");
   EnterTabletMode();
   // DisplayConfigurationObserver enables mirror mode when tablet mode is