Changing IdentityManager::Observer and its subclasses to use CoreAccountId

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

Bug: 959157
Change-Id: I0fd318d8034fb1294336f7534eb672fd1faeb084
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611996
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
Cr-Commit-Position: refs/heads/master@{#661344}
18 files changed