Android: Close preview panel on some events

Preview (a.k.a Sneak Peek) panel needs to be closed on various events
such as when a new tab is opened, or its base panel navigates, etc.
This CL adds a couple of tab model observers to detect those events,
and invokes |closePanel| accordingly.

Bug: 940919
Change-Id: Ieee694db214ded1f031ac24db3586496d19bee42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1519966
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641089}
4 files changed