[WICG/webcomponents] Add delegatesLabel and a content attribute to specify the label element within a shadow tree (#916)

During AOM F2F today, a common use case of wanting to label a custom element but want that label to activate a specific element within its shadow root when the label is to be activated, and the assistive technologies to label that specific element in the shadow tree instead of the entire shadow host or its shadow tree in its entirety.

We came up with the idea of `delegatesLabel` which like `delegatesFocus` will delegate the label specified on a shadow host to a specific element within the shadow root. In order to define which element will get labeled on behalf of the shadow host, a content attribute (e.g. `autofocus` or `labelfor`) should also be added.


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https://github.com/WICG/webcomponents/issues/916