Revert "Changing OAuth2TokenService::Observer and its subclasses to use CoreAccountId instead of std::string"

This reverts commit 2669b5b556b8d0c7d0915a6cd3cbb1f597489999.

Reason for revert: I believe this one is responsible for compile failure: https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8912567103159431936/+/steps/compile/0/stdout

Original change's description:
> Changing OAuth2TokenService::Observer and its subclasses to use CoreAccountId instead of std::string
> 
> This cl is part of the effort to use CoreAccountId instead of std::string
> to denote account_id.
> 
> Bug: 959157
> Change-Id: I6b95b6adf7d73228c399162daf2867c532acd7cd
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626599
> Reviewed-by: David Roger <droger@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
> Cr-Commit-Position: refs/heads/master@{#663069}

TBR=droger@chromium.org,sdefresne@chromium.org,triploblastic@google.com

Change-Id: I716aa951acef794062444f73954083e5b0981c29
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 959157
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628724
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663081}
17 files changed