element.getContentChildNodes generates exception under Shadow dom when a content is not found

[sorvell]

The element.getContentChildNodes(selector) generates an exception when run under Shadow DOM when the selector argument does not correspond to an existing <content> element. When run under Shady DOM, this returns an empty array. We should normalize this behavior.

The root cause is that Polymer.dom(element).getDistributedNodes generates an exception under Shadow DOM when element does not have this api. This should be normalized as well.