DevTools: Extract shortcut handling from KeyboardShortcut.js into ShortcutRegistry.js

This patch prepares the code for handling of shortcuts forwarded from the inspected page.

R=pfeldman@chromium.org, pfeldman, vsevik

Review URL: https://codereview.chromium.org/242853002

git-svn-id: svn://svn.chromium.org/blink/trunk@171937 bbb929c8-8fbe-4397-9dbb-9b2b20218538