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On a 401 (HTTP unauthorized) error, the correct response it to get a
new access token and try again (most likely reason for the error is that
the previous access token was expired).
PerUserTopicSubscriptionManager did this, but SubscriptionEntry's
SubscriptionFinishedCallback was a OnceCallback, and it wasn't re-set
for the second subscription attempt. So the result of the second attempt
was simply ignored: If it succeeded, we'd now be subscribed, but not
know about it (likely resulting in a redundant subscription request at
some later time). If it failed, it wouldn't get retried anymore, so
invalidations would be broken until at least the next Chrome restart.

This CL fixes this by simply making the callback a RepeatingCallback. It
also adds a test.

Bug: 1020117
Change-Id: Ifaf29a6db21ce9f37888603b534ef50970ffc94f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000720
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Tim Schumann <tschumann@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732055}