DevTools: [CodeMirror] fix usage of findmatchingbracket addon

The matchingbracket addon API changed after the roll; this is a
follow-up patch to fix its client-side call.

BUG=360497
TBR=apavlov, vsevik

Review URL: https://codereview.chromium.org/227483005

git-svn-id: svn://svn.chromium.org/blink/trunk@170964 bbb929c8-8fbe-4397-9dbb-9b2b20218538