Don't access front() if the queue is empty.
Don't access front() if the queue is empty.

TEST=This fixes a crash that I ran into when testing CF.  I was using this page: http://testbed.nicon.nl/googlechrome/testcase_googlechrome.asp?cnt=plus
BUG=38547

Review URL: http://codereview.chromium.org/1107005

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@42054 0039d316-1c4b-4281-b951-d872f2087c98

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