DevTools: Add Cmd-\ as a pause shortcut

This patch assigns Cmd/Ctrl-\ for pause/continue shortcut.

BUG=246447

Review URL: https://chromiumcodereview.appspot.com/22880014

git-svn-id: svn://svn.chromium.org/blink/trunk@156269 bbb929c8-8fbe-4397-9dbb-9b2b20218538