[webcomponents] A negative tabindex on shadow host should be respected (#399)

Both in the [current specification](https://www.w3.org/TR/shadow-dom/#focus-navigation) and in [the proposal](https://docs.google.com/document/d/1mNV2Z231SrAZuMJvBVNfVNHfULkMwUZOZ75vZM1K9zY/edit) in the issue #381, contents inside a shadow tree is treated as if its shadow host has the tabindex of 0.

However, when the shadow host has a negative tabindex, its contents should be not be in the sequential focus navigation order.

The `input` element doesn't place focus in its editable region when it has a negative tabindex for example, and it's very counterintuitive for the tabindex on an element to be ignored just because it contains a focusable element.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/399