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Some of the chrome/browser/policy code will be moved to a new layered component.
This change prepares the ConfigurationPolicyPrefStore class for the move, by
removing its dependency on g_browser_process and BrowserThread.

BUG=271392

Review URL: https://chromiumcodereview.appspot.com/22812002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@217084 0039d316-1c4b-4281-b951-d872f2087c98