Support author ShadowRoot for APPLET, EMBED, and OBJECT elements.

The behavior is similar to INPUT.  That is to say, we don't allow
<shadow> projecting UA-native view, and an element loses native
behavior if an author ShadowRoot is added.

When an author ShadowRoot is appended, we make  HTMLPlugInElement::
useFallbackContent return true to avoid plugin processing, and request
to reattach.
We add UA ShadowRoot to show a warning about inert <shadow> to
developers.

BUG=301217
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/68133007

git-svn-id: svn://svn.chromium.org/blink/trunk@163140 bbb929c8-8fbe-4397-9dbb-9b2b20218538