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Tab sync entities are expected to have a valid |tab_id| field over the
sync protocol, but clients should not crash if the received data is
corrupt, e.g. committed by a buggy client.

Prior to this patch, foreign sessions received in SessionsSyncManager,
processed via UpdateTrackerWithSpecifics(), could trigger a CHECK
failure in SyncedSessionTracker::GetTab().

Bug: 840876
Change-Id: Ie59cfb9c24934695a1b7b6187dbad48451f8c8ea
Reviewed-on: https://chromium-review.googlesource.com/1051233Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557124}