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This reverts commit 58fe1e69.

Reason for revert: This looks like it breaks an ios internal trybot https://ci.chromium.org/p/chrome/builders/luci.chrome.ci/ios-internal-chromium-tot

Original change's description:
> Delete GaiaCookieManagerServiceObserver in favor of callbacks.
> 
> 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/+/1630162
> Reviewed-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}

TBR=blundell@chromium.org,msarda@chromium.org,lowell@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 939372
Change-Id: I777df13eb9bc5eddeea46640b973c287d6622061
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642232Reviewed-by: Lowell Manners <lowell@chromium.org>
Commit-Queue: Lowell Manners <lowell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666007}