Delay firing iron-iconset-added until iconset fully parsed.

[andrey-git]

On Chrome a custom element constructor and lifecycle callbacks are called as soon as the tag itself is parsed, i.e. <iron-iconset-svg ... > but possibly before the child elements are parsed.

This PR handles this case by waiting for DOMContentLoaded before firing iron-iconset-added if the document is in loading state

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[andrey-git]

I signed it!

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