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Before this CL, when FCMInvalidationServiceBase triggered an InstanceID
(aka client ID) deletion, it didn't actually clear its |client_id_|
member, and it didn't propagate the deletion to its clients (via the
InvalidatorRegistrar).
This CL fixes these two things.

This *probably* didn't really have consequences in practice: Only the
SyncEngine actually used the InstanceID (for reflection blocking), and
in this case it anyway gets shut down as well. Still, let's not keep
obsolete IDs around.

Bug: 1028761
Change-Id: I5c09aff83845bebab1b4fb6b7d8fd963ae91501f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1954475
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Tim Schumann <tschumann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723413}