DevTools: Implement extensions-based shortcut bindings

R=eustas, pfeldman, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/170273003

git-svn-id: svn://svn.chromium.org/blink/trunk@170156 bbb929c8-8fbe-4397-9dbb-9b2b20218538