[scoped-registry] Allow attaching a registry when creating a shadow root

This patch adds a `registry` parameter to ShadowRootInit so that we
can attach a registry when creating a shadow root. It also adds a
`ShadowRoot.registry` readonly attribute to return the attached registry
of a shadow root.

Bug: 1304439
Change-Id: I0514cd9d25d6fab2696b468ab0d066677bd66e6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3915480
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1053679}