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As a first step toward completely eliminating the usage of
ProfileIdentityProvider in //components/gcm_driver, this CL threads in
SigninManager and replaces direct usage of ProfileIdentityProvider in
GCMProfileService::IdentityObserver with equivalent usage of
SigninManager.

Note that it is easy to verify that the usage is equivalent:
The ProfileIdentityProvider method calls are replaced with their
implementations in profile_identity_provider.cc.

Full design doc here:
https://docs.google.com/document/d/1OmNrIiMDkF7eOYVB4RIiDvY7pQz3zUrOf-D4TVwdRQA/edit?ts=5aa6b936#

TBR=zea@chromium.org, rockot@chromium.org

Bug: 809923
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I20515d7431f0e8f5370a47cd04c8b21eb0874d6f
Reviewed-on: https://chromium-review.googlesource.com/1004999
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556408}