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This CL restricts the AccountTracker public API to what's actually
used by GCMAccountTracker, its only client. This helps to clarify what
the purpose of AccountTracker is.

The CL also removes an AccountTracker unittest that was calling into a
now-private API. That test checks that a token revocation doesn't
cause any AccountTracker event to fire if the token was never made
available in the first place. That sequence of events (token revocation
being fired by OAuth2TokenService without the token first being made
available) doesn't reflect reality, so there is no purpose in testing
for it.

Change-Id: Ic3085897b87895faa6cf0c3d5d5d9f6520d0c808
Reviewed-on: https://chromium-review.googlesource.com/1046832
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561013}