Move IdentityManager to //components/signin

IdentityManager currently lives under //services/identity/public/cpp. Given
the evolution of the Identity Service which is backed by IdentityManager, with
IdentityManager being used directly by the browser, this code location no
longer makes sense.

This CL performs the following relocations:
* Public header files (e.g. identity_manager.h, access_token_fetcher.h), their
  implementations, and their unittests will move to
  //components/signin/public/identity_manager.
* Non-public headers, their implementations, and their unittests will move to:
  //components/signin/internal/identity_manager.

Design doc:
https://docs.google.com/document/d/1MiZmqvDFVcCOseAuViJqd8wLDILbykarX7PfwleNpAs

[email protected],[email protected],[email protected]

Bug: 952788
Change-Id: Ied8992f966f7373443fbb76b177ad4d71832145d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669293
Reviewed-by: Henrique Ferreiro <[email protected]>
Reviewed-by: Colin Blundell <[email protected]>
Commit-Queue: Henrique Ferreiro <[email protected]>
Cr-Commit-Position: refs/heads/master@{#674455}
diff --git a/components/gcm_driver/account_tracker.h b/components/gcm_driver/account_tracker.h
index bcfca70..1e2e65ed 100644
--- a/components/gcm_driver/account_tracker.h
+++ b/components/gcm_driver/account_tracker.h
@@ -12,9 +12,9 @@
 
 #include "base/memory/scoped_refptr.h"
 #include "base/observer_list.h"
+#include "components/signin/public/identity_manager/access_token_fetcher.h"
+#include "components/signin/public/identity_manager/identity_manager.h"
 #include "google_apis/gaia/gaia_oauth_client.h"
-#include "services/identity/public/cpp/access_token_fetcher.h"
-#include "services/identity/public/cpp/identity_manager.h"
 
 class GoogleServiceAuthError;