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* As of today, when subscription to the topic gets deleted,
the value in prefs is cleaned up only after the response for HTTP
unsubscription request arrives.
* It can happen, e.g. on the browser shutdown that the clean-up
code is never executed, hence prefs won't be cleaned up.
* This CL makes the prefs cleanup happen when the decision to unsubscribe
from the topic notifications is made.

BUG=913486

Change-Id: I3d58df8896ef87ecef4535fb588d9a73494c2de1
Reviewed-on: https://chromium-review.googlesource.com/c/1370164
Commit-Queue: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615852}