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This fixes one (small) issue where the next_token_request_time didn't
get cleared if the retry timer called us while there was already a
request ongoing. It also adds a bunch of DCHECKs to hopefully catch
any other cases I might have missed.
Inspired by some recent bug reports that showed a "Next Token Request"
time in the past on about:sync-internals.

Bug: 889859
Change-Id: I8cd189c0e4acccc4af0d38a3ce9da6ccbd3cac7a
Reviewed-on: https://chromium-review.googlesource.com/1249067Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594714}