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On Android, Sync only subscribes for SESSIONS invalidations while the
Recent Tabs page is open.

Before this CL, the corresponding *un*subscription was delayed by one
hour, to avoid subscription/unsubscription churn. However, this task
was not persisted, i.e. it got lost if Chrome was shut down within that
hour. The new FCM-based invalidations system does not do any cleanup on
startup (in contrast to the old Tango system), meaning such an
unsubscription would then never actually happen.

This CL removes the unregistration delay, as a quick fix to avoid most
of the "dangling subscriptions". It also adds a feature
"SyncUseSessionsUnregisterDelay" (disabled by default) to go back to
the previous behavior.

Bug: 1024817
Change-Id: Iaaf1afca3ba8719a3837dd760e4bd169d4d3c3c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919209Reviewed-by: Tim Schumann <tschumann@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719621}