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StopListeningPermanently() gets called during sign-out. The FCM token
needs to be cleared so that the change can be propagated to device info.
Empty token in device info signals that the device is not listening to
invalidations.

Change-Id: I6d0a4e2007af6d7457cafed265877ec9810cb4f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2465844
Commit-Queue: Paula Vidas <paulavidas@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/master@{#816173}