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Prior to this CL, calling imperative slot API does not check the
validity of the slot element or its assigned nodes. However, after
the last TPAC Meeting,
https://github.com/whatwg/html/issues/3534#issuecomment-537802687,
the members agree to not allow manual assignment of any light-tree
descendant other than the host's direct light-tree children.

This CL validates the slot element and its assigned nodes when calling
imperative slot API. The checks are: shadow root is V1, shadow root is
in manual slot assignment mode, and assign nodes are children of slot's
host element.

Bug: 869308
Change-Id: I4b0a1a1a3af6bf159e22af5eafe32ed44172b021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2096958Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Yu Han <yuzhehan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750637}