Fixed modality of port-forwarding dialog in chrome://inspect

BUG=301591

Review URL: https://codereview.chromium.org/28653004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230192 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230192