GitLab

- keyboard overlay and partial_screenshot overlay should do nothing
  if there are already overlays.
- overlay_event_filter should cancel the existing delegate if
  another delegate is trying to activate.

BUG=341958
R=jamescook@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/461093003

Cr-Commit-Position: refs/heads/master@{#289199}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289199 0039d316-1c4b-4281-b951-d872f2087c98