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This CL replaces GaiaCookieManagerServiceObserver with vanilla
callbacks. Now that GCMS is owned by IdentityManager, and cannot have
multiple observers, a callback interface seems more natural than an
Observer class.

This also has the nice side effect of removing transitive includes of
gaia_cookie_manager_service.h via identity_manager.h.

Change-Id: I9cdac56c8237c9a09eb2c2ce87f41f48872a9fb6
Bug: 939372
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1630162Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Lowell Manners <lowell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665616}