Fix underflow that would cause either an access violation or an infinite loop if we were ever to get a "chrome_config" node back in metrics response data.

Review URL: http://codereview.chromium.org/42335

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@11983 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed