Ensure newly created templates have content

[kevinpschaaf]

Part of fix for Polymer/polymer#1500

[dfreedm]

My only concern is that this will break the decorate function for templates that were created by script before DOMContentLoaded.

@garlicnation thoughts?

[kevinpschaaf]

@azakus the content created by decorate at DOMContentLoaded won't stomp on content created by this createElement addition (it existence checks here: https://github.com/webcomponents/webcomponentsjs/blob/ensure-tmpl-content/src/Template/Template.js#L28).

[kevinpschaaf]

I'll make a change so that template children are moved into content even when the template is created imperatively, so that you could innerHTML = ... an imperatively-created template and have decorate move them into the content at DOMContentLoaded time.

[garlicnation]

Merged.