DevTools: layer panel stub -- raw, ugly and useless.

Added boilerplace code for layers panel, plus a sidebar tree with layers.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/22576006

git-svn-id: svn://svn.chromium.org/blink/trunk@156005 bbb929c8-8fbe-4397-9dbb-9b2b20218538