[webcomponents] [Shadow] ShadowRoot object may contain encapsulation mode (#275)

Whether or not a shadow root is created 'open' or 'closed' cannot be distinguished
because no information is left in shadow root object.

A workaoround might be checking if ```shadowRoot.host.shadowRoot === shadowRoot```
where ```shadowRoot``` is a ShadowRoot object.

Instead, shall we have a read-only ```.mode``` attribute which reflects the parameter
given at ```createShadowRoot()```?


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/275