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to LoginUIService. This will be used by the extension API to show
login UI in a popup and then obseve when it is closed and respond back
to the extension accordingly.

Review URL: https://chromiumcodereview.appspot.com/10699013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@146143 0039d316-1c4b-4281-b951-d872f2087c98