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This reverts commit 7a9ca38d.

Reason for revert: It appears all the browserlist observers are
busted after closing the last incognito tab.

Original change's description:
> [iOS] Create WebStateList observation registrar.
> 
> This CL creates AllWebStateListObservationRegistrar, which can be used
> in any case when an object needs to observe all web state lists for a
> given BrowserState.
> 
> This is a generalized version of the snapshot cache tab model observer,
> which this replaces.
> 
> For cases where only regular or OTR browsers' WebStateLists should be
> observed, AllWebStateListObservationRegistrar has an optional param that
> can be set at creation time.
> 
> To avoid circular dependencies, one target in web_state_list/ had to
> be split from the main target (hence the scattershot BUILD.gn updates).
> 
> Change-Id: Ic3977b709d5b089ebf31afd0fa868067833f2249
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1997306
> Commit-Queue: Mark Cogan <marq@chromium.org>
> Reviewed-by: Rohit Rao <rohitrao@chromium.org>
> Reviewed-by: Justin Cohen <justincohen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#742172}

TBR=rohitrao@chromium.org,marq@chromium.org,justincohen@chromium.org

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

Change-Id: If97541ae3196c04da57c8a49896e8c55d5646949
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063918Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743107}