Custom Elements: Add show() methods for queues and steps.

This change adds a helper functions for interactive debugging.

TEST=none
BUG=none
R=dominicc@chromium.org

Review URL: https://codereview.chromium.org/269633002

git-svn-id: svn://svn.chromium.org/blink/trunk@173132 bbb929c8-8fbe-4397-9dbb-9b2b20218538