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This CL separates separates the mechanism via which
SafeBrowsingPrimaryAccountTokenFetcher fetches access tokens and the
mechanisms via which it tracks and responds to ongoing token fetches,
moving the latter into a new SafeBrowsingTokenFetchTracker helper class
for reuse by WebLayer. This class includes the following:

- Tracking of ongoing access token fetch requests coming from the client
  of SafeBrowsingTokenFetcher, responding to these requests when
  notified that a given access token has been fetched, and gating these
  requests on a given time threshold (responding with an empty access
  token if a request times out)
- Responding to all active token requests with an empty access token on
  destruction

This CL also adds unittests of SafeBrowsingTokenFetchTracker.

Via this helper class, WebLayer will be able to share the key
functionality of gating safe browsing access token fetches on a timeout
rather than duplicating this functionality and risking divergence over
time.

The only subtlety is the behavior on token fetch timeout:
SafeBrowsingPrimaryAccountTokenFetcher needs to be notified in addition
to its client so that the former can destroy the corresponding
AccessTokenFetcher. To facilitate this,
SafeBrowsingTokenFetchTracker::StartTrackingTokenFetch() takes in both
the SafeBrowsingTokenFetcher::Callback that the client passed in and a
callback to be invoked on timeout, via which its owner can clean up any
state associated with the request.

Bug: 1080748
Change-Id: I79bb83096cc2094c7cf9916eccf382271a5f46a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624630
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843441}