GitLab

Split CustomTabActivity's PageLoadMetrics.Observers according to their
purpose. Also create the TabObserverRegistrar class that deals with
attaching these observers to Tabs as they enter and exit the TabModel.

Bug: none
Change-Id: Ic434ead9c5167eb876f91af33b54dfd3d50493cd
Reviewed-on: https://chromium-review.googlesource.com/1154968Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580474}