Defensive checks for backend_ existence. BUG=3231 (chromium-os) http://code.google.com/p/chromium-os/issues/detail?id=3231
Defensive checks for backend_ existence.
BUG=3231
(chromium-os)
http://code.google.com/p/chromium-os/issues/detail?id=3231

Review URL: http://codereview.chromium.org/2058006

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@47156 0039d316-1c4b-4281-b951-d872f2087c98

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