[w3c/webcomponents] Form autocompletion is broken when form elements are encapsulated within a Shadow DOM (#649)

HTML forms have a number of features based on association between controls and forms. To name a few: [form autofill](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#autofill), [implicit submit](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#implicit-submission) and [form submission](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#form-submission) in general.

As browsers start to ship native Shadow DOM implementations it becomes apparent that Shadow DOM is not designed to work well with these HTML form features. E.g. it has been already noted in this repo, that the way how forms submission works with custom elements should be improved (see the issue #187).

This issue is to improve the web components specs in the part of _forms autofill_ and _implicit submit_. Please either specify how that should work across Shadow DOM boundaries, or make an explicit statement that these features of HTML forms are not expected to work with Shadow DOM.

Here is a live demo for this issue: https://github.com/vlukashov/polymer-forms. It shows that in the browsers that support the native Shadow DOM (Chrome, Safari) neither _forms autofill_ nor _implicit submit_ work as expected.

However, without a spec it is difficult to call this behavior a browser bug.

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https://github.com/w3c/webcomponents/issues/649