DevTools: Always construct CodeMirror explicitly, via "new"

This patch also cleans up and fixes a bunch of codemirror-related externs

R=lushnikov
NOTRY=true

Review URL: https://codereview.chromium.org/334393004

git-svn-id: svn://svn.chromium.org/blink/trunk@176332 bbb929c8-8fbe-4397-9dbb-9b2b20218538