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/+/1827501


Reviewed-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}