ElementInternals: Implement disabledFeatures=['shadow']

Add support of disabledFeatures=['shadow'] in a custom element
definition. It disables Element.attachShadow().

Specification PR: https://github.com/whatwg/dom/pull/760
The behavior is behind "ElemnetInternals" runtime flag.

Bug: 905922
Change-Id: Ib950883b5c995945eb60bf9b92c715ad3996dfa4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609022
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661327}
4 files changed