Changing ProfileOauth2TokenService to use CoreAccountId to denote account_id

This cl is part of the effort to use CoreAccountId instead of std::string
to denote account_id.

Bug: 959157
Change-Id: I6de67955e067930b59b0f3e8617cfcfcf8b0de2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1655475
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
Cr-Commit-Position: refs/heads/master@{#669205}