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The class is a key piece in the design doc ([1], accessible for Google
employees only) and will be used by the future USS bridge, to be
introduced in follow-up patches.

Ideally, SessionStore is an abstraction on top of SyncedSessionTracker
that in addition supports [de]serialization to/from disk. In the current
form, in diverges slightly from the design doc because direct mutable
access to the underlying SyncedSessionTracker is exposed. TODOs have
been added to address this in the future.

[1] https://docs.google.com/document/d/1UGM1yhHznmXvfeGvckdzBcrE6GLpgk7LsLmgPhFcIdk/edit#

Bug: 681921
Change-Id: Ia483faf7256b5c6296f33236b42974697f4e6f4e
Reviewed-on: https://chromium-review.googlesource.com/973222
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550587}