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https://codereview.chromium.org/2673443002 introduced a bug in the
Identity extension API implementation: it assumed that there would be
only one IdentityGetAuthToken instance that needed to listen for
IdentityAPI shutdown at a given time, but in fact, there can be many.
An example is if the user has multiple tabs open that each call the
getAuthToken() function with interactive set to true, so that they
end up hanging waiting for the user to authenticate.

The result of this bug is that if the browser shuts down with multiple
getAuthToken() calls active, only the last call will receive notice of
IdentityAPI shutdown. Other calls can then end up calling back into
Profile during its shutdown process, e.g. if they get notified by
OAuth2TokenService as part of *its* shutdown process that it has failed
their token requests. These calls can result in CHECKs (see crash
reports linked from crbug.com/720073).

This CL fixes the bug while maintaining the property that
IdentityGetAuthToken does not require an interface dependence on
IdentityAPI (i.e., the property that was the motivation of the original
CL). To do so, it changes IdentityAPI to maintain a CallbackList that
gets invoked on its shutdown.

This CL also introduces a browsertest that fails without this change.

Bug: 720073
Change-Id: I44daaa0c0bbc27e54da9ab826e256c70f3d5a2a8
Reviewed-on: https://chromium-review.googlesource.com/680974Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504289}