DevTools: move layers subsystem off WebInspector.domModel. Also random drive-by domModel removals.

R=caseq@chromium.org

Review URL: https://codereview.chromium.org/216823007

git-svn-id: svn://svn.chromium.org/blink/trunk@170849 bbb929c8-8fbe-4397-9dbb-9b2b20218538