DevTools: Implement extension-based status bar buttons

This allows to extract code from "core" into "settings"

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/283063003

git-svn-id: svn://svn.chromium.org/blink/trunk@175358 bbb929c8-8fbe-4397-9dbb-9b2b20218538