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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}