Make it possible to condition an element type on context features.

Any such tag name would need to also set noTypeHelpers: true and
provide its own specializations that check something besides the
tag name, since such an element could still end up in a document
where it is disabled through node adoption.

Bug: 1040627
Change-Id: Ib53554f673e2a7b375e82abde1b8e29c380c39c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994520
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734491}
8 files changed