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This renaming puts the Signin embedder interface in line with the agreed-upon
convention wherein a component Foo's embedder interface should be named
FooClient. ChromeSigninManagerDelegate is similarly renamed to
ChromeSigninClient, and variable names are updated.

BUG=334168
TBR=joi

Review URL: https://codereview.chromium.org/192253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255903 0039d316-1c4b-4281-b951-d872f2087c98