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This reverts commit 241c941f.

Reason for revert: Causes crash: crbug.com/875697

Original change's description:
> Record discarded state in TabActivityWatcher.
> 
> (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/1166763
> Reviewed-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}

TBR=chrisha@chromium.org,jam@chromium.org,michaelpg@chromium.org,holte@chromium.org,charleszhao@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 872139
Change-Id: Ic32c319d1a75ebf3237677ccb8935744f95dfae2
Reviewed-on: https://chromium-review.googlesource.com/1181586Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584540}