Have Identity extension API impl find accountID via AccountTrackerService

As crbug.com/729590 details, we are looking to eliminate AccountTracker
entirely in favor of AccountTrackerService. As a step in that direction,
this CL eliminates the one usage of
AccountTracker::FindAccountIdsByGaiaId(), replacing it with equivalent
usage of AccountTrackerService. It updates the corresponding browsertests
to interact with AccountTrackerService rather than AccountTracker as well.

Note that both AccountTracker and AccountTrackerService add an
account in response to receiving OAuth2TokenService::OnRefreshTokenAvailable(),
and both populate it with a GAIA ID in response to the user's info being
fetched. Hence this change should be a no-op behaviorally.

This CL also removes the now-unused function.

Bug: 729590, 729582
Change-Id: Ideb1c4f0398d662ae6d0db04872474a52e3ef21d
Reviewed-on: https://chromium-review.googlesource.com/535524
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#480001}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fd8e7a2b81bcf21148850baf3cb77a299a1bc185
3 files changed