GitLab

- Adds OverlayPresenter::Observer::RequestCancelledByQueue().
  OverlayPresenterImpl uses this callback to cancel overlay UI for
  requests cancelled by the queue.
- Adds OverlayRequestQueueImpl::RequestCanceller, which uses
  WebStateObserver callbacks to cancel requests in a queue for
  navigation events.

Bug: 941745
Change-Id: Ia5f78e22fe4a0397aa1530bf41cb0fb1b434e398
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598287
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659524}