Remove UserAgent ShadowRoot + Author ShadowRoot code.

The code is not used since crrev.com/378921.
This CL removes Element::willAddFirstAuthorShadowRoot, which was added to
support this feature.

This CL adds ShadowRootType stream printer because ShadowRootType is used in
DCHECK_NE().

BUG=489947

Review URL: https://codereview.chromium.org/1901403002

Cr-Commit-Position: refs/heads/master@{#388634}
18 files changed