[cros] Make Authenticator instance scoped_refptr again, fix crash.
[cros] Make Authenticator instance scoped_refptr again, fix crash.

Other users of the Authenticator instance store it in scoped_refptr (LoginPerformer, ScreenLocker).

BUG=chromium-os:18200
TEST=Signed in, observed no crash.

Review URL: http://codereview.chromium.org/7461076

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@94088 0039d316-1c4b-4281-b951-d872f2087c98

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