DevTools: [wip] move Elements panel off WebInspector.domModel and single tree outline.

R=sergeyv@chromium.org

Review URL: https://codereview.chromium.org/218703002

git-svn-id: svn://svn.chromium.org/blink/trunk@170453 bbb929c8-8fbe-4397-9dbb-9b2b20218538