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This CL adds
CustomTabActivityTabProvider#registerTabObserver()
CustomTabActivityTabProvider#unregisterTabObserver()
These methods register (and unregister) a TabObserver for the
CustomTabActivity's active tab. The tab being observed is changed when the
CustomTabActivity's active tab changes. This differs from
TabObserverRegistrar#registerTabObserver() which registers the passed-in
TabObserver for all newly created tabs.

BUG=1020911
TEST=TabObserverRegistrarTest.testObserveActiveTab

Change-Id: Ie5c4b8f287209b672e89184a3e9f0710c69102ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1896140
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713554}