Better focus support for Shadow DOM

Hi,

For shadow DOMs which has multiple focusable fields under the host,
the current behavior of tab navigation order gets somewhat weird
when you want to specify tabindex explicitly.

This is the doc to introduce a new attribute "delegatesFocus" to resolve
the issue.
https://docs.google.com/document/d/1k93Ez6yNSyWQDtGjdJJqTBPmljk9l2WS3JTe5OHHB50/edit?usp=sharing

Any comments are welcome!
-- 
Takayoshi Kochi