DevTools: make some SDK objects target-aware, remove mainTarget use and some WebInspector.*Model accessors.

R=sergeyv@chromium.org

Review URL: https://codereview.chromium.org/218623002

git-svn-id: svn://svn.chromium.org/blink/trunk@170439 bbb929c8-8fbe-4397-9dbb-9b2b20218538