GitLab

(1) TabLifecycleObserver is added for TabActivityWatcher to track
    OnDiscardedStateChange event.

(2) In DidReplace(replaced_tab), the ukm_source_id and is_discarded
    state is copied to new_tab so that we can track discarded state
    after reload or closing.
(3) A browser test is added.

Change-Id: I9777b515d28d215f3409525b0201dc5916d35941

Bug: 872139
Change-Id: I9777b515d28d215f3409525b0201dc5916d35941
Reviewed-on: https://chromium-review.googlesource.com/1166763Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584217}