DevTools: [CodeMirror] support undo last selection command

This expands "undo last added selection" command into CodeMirrorTextEditor via Ctrl-U shortcut.
This is useful when you create more selections then needed with
Ctrl-D command.

Review URL: https://codereview.chromium.org/233073004

git-svn-id: svn://svn.chromium.org/blink/trunk@171310 bbb929c8-8fbe-4397-9dbb-9b2b20218538