DevTools: Make ShortcutRegistry operate in terms of actionIds only

This is a preparatory step for the implementation of shortcut overrides

R=eustas, pfeldman

Review URL: https://codereview.chromium.org/264583002

git-svn-id: svn://svn.chromium.org/blink/trunk@174155 bbb929c8-8fbe-4397-9dbb-9b2b20218538