Added CoreAccountId in mojom files

This cl is part of the effort to introduce strongly typed CoreAccountId
instead of std::string in CoreAccountInfo to stop confusion among
gaia_id, email and account_id.

Bug: 959157
Change-Id: I78c1c77b9a5634d03154cfc4b8a95e517aa73620
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602647
Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659585}
16 files changed