DevTools: [CodeMirror] bring back Ctrl-Y shortcut

This patch adds Ctrl-Y shortcut for the redo to the PC keymap.

BUG=

Review URL: https://chromiumcodereview.appspot.com/18212002

git-svn-id: svn://svn.chromium.org/blink/trunk@153299 bbb929c8-8fbe-4397-9dbb-9b2b20218538