Remove the support of multiple shadow roots with a user agent shadow root

"Intent to Remove" is here: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/VKQk3uQuJLQ

This CL removes the web-facing API, as a first step.
The other removals, such as a cleanup of each element, will be done separately in other CLs.

BUG=489947

Review URL: https://codereview.chromium.org/1751953003

Cr-Commit-Position: refs/heads/master@{#378921}
82 files changed