DevTools: Fix exception in inspect.js when saving port forwarding settings

TBR=pfeldman
BUG=318494

Review URL: https://codereview.chromium.org/70513006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234785