[w3c/webcomponents] Shadow host with its .tabindex property -1 would be skipped for focus navigation (#556)

Context:
https://github.com/w3c/webcomponents/issues/554#issuecomment-242636505

In the Shadow DOM spec 6.3
http://w3c.github.io/webcomponents/spec/shadow/#sequential-focus-navigation
specifically in Step 3.1.3, all elements with negative tabindex value will be skipped in the focus navigation, but shadow hosts with `delegatesFocus=faluse` should be excluded otherwise all focusable elements in its shadow root will never visited.

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