DevTools: preload codemirror for edit-dom-actions test

The patch preloads "sources" panel in edit-dom-actions initializer in
order to support "Edit as HTML" action.

R=pfeldman

Review URL: https://codereview.chromium.org/675473002

git-svn-id: svn://svn.chromium.org/blink/trunk@184223 bbb929c8-8fbe-4397-9dbb-9b2b20218538