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While working on GaiaCookieManagerService, I noticed that
SigninClient::AddCookieChangeCallback() is unnecessary to call out to
the embedder for: the only embedder-specific parameterization of the
//components-level SigninCookieChangeSubscription is for the
URLRequestContextGetter, which is already available to the component
via SigninClient::GetURLRequestContext().

This CL eliminates that client method in favor of having
GaiaCookieManagerService construct SigninCookieChangeSubscription
directly. I verified that all production clients were supplying the same
URLRequestContextGetter that they return in their implementation of
SigninClient::GetURLRequestContext().

TBR=jzw@chromium.org

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ic8a9d0c6e3e63cf531408eb6ab46db65bcf3e668
Reviewed-on: https://chromium-review.googlesource.com/1145312
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577467}