Clean up about box for unofficial builds.
Clean up about box for unofficial builds.

Currently unofficial builds (Chromium) display there an error message
that update server is unavailable. This is obvious (there is no update
server for trunk builds), but looks bad. This patch removes
the message for unofficial builds.
Review URL: http://codereview.chromium.org/13352

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@6806 0039d316-1c4b-4281-b951-d872f2087c98

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