DevTools: Migrate zooming and "Inspect element" mode toggling shortcuts to extensions

R=vsevik@chromium.org, eustas, pfeldman
BUG=174309

Review URL: https://codereview.chromium.org/242603003

git-svn-id: svn://svn.chromium.org/blink/trunk@172027 bbb929c8-8fbe-4397-9dbb-9b2b20218538