Stabilize the behavior of platform views in Flutter web.

Unify how platform views are rendered in the web for both rendering
 backends (html and canvaskit).

Provide a predictable location in the DOM that allows developers to reliably
 use CSS to style their platform views, and to use other standard DOM API,
 such as querySelector, and getElementById.

Description of change

A Flutter web app is now rendered inside a common shadow root in which
slot elements represent platform views. The actual content of
each platform view is rendered as a sibling of said shadow root.

Before

<flt-glass-pane>
  ...

<div

id=

"platform-view"

>Contents

</div>

<!-- canvaskit -->

<!-- OR -->

<flt-platform-view>
    #shadow-root
    |