GitLab

Make it a pure virtual interface. Per the style guide, classes should
have no more than one superclass that actually provides implementation.
This fixes BrowserView in that regard. This is also necessary for
upcoming ScopedObserver updates.

Bug: 998625
Change-Id: I387e2ef98af0035023a72853b10cc5fba8db1df4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1828280Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700822}