[webcomponents] Introduce "delegatesFocus" flag to shadow root (#126)

To fix focusability and focus navigation order issue ([W3C 25473](https://www.w3.org/Bugs/Public/show_bug.cgi?id=25473), [Chromium 380445](https://code.google.com/p/chromium/issues/detail?id=380445)), introduce ```delegatesFocus``` flag under shadow root, to address some shortcomings that
the default focus navigation behavior ([defined in the Shadow DOM spec](https://w3c.github.io/webcomponents/spec/shadow/#focus-navigation)) has.

Detailed doc is here:
https://github.com/TakayoshiKochi/tabindex-focus-navigation-explainer/blob/master/TabindexFocusNavigationExplainer.md

I'll post the proposal doc soon.
This is experimentally implmented in Blink [Chromium 496005](http://crbug.com/496005)

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/126