Private API for web store to get sync login and set store login information.

      BUG=53489
TEST=Not sure yet the best way to test this - it will depend on how the store
implements their part of the API. 

Review URL: http://codereview.chromium.org/3219006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@58126 0039d316-1c4b-4281-b951-d872f2087c98