Fix a CHECK() being hit when the safebrowsing check times out (passing in empty URL but expecting the deferred URL).

      BUG=37195,33572
TBR=wtc
Review URL: http://codereview.chromium.org/660396

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@40426 0039d316-1c4b-4281-b951-d872f2087c98