DevTools: [CodeMirror] use Cmd-D instead of Ctrl-D on Mac

This patch makes "select next occurrence" shortcut to be Cmd-D/Cmd-U on
Mac instead of Ctrl-D/Ctrl-U.

Review URL: https://codereview.chromium.org/242943002

git-svn-id: svn://svn.chromium.org/blink/trunk@171941 bbb929c8-8fbe-4397-9dbb-9b2b20218538