Implement imperative slotting API changes

[chromium-wpt-export-bot]

The original implementation of the imperative slot distribution
API was done before the final spec PRs landed. In the process of
landing those PRs, several changes were made to the way the API
works. Primarily, there are two changes:

1. The "auto" slotAssignment mode was renamed to "named".
2. The "linkage" that is created by HTMLSlotElement.assign() was
   made more permanent. Previously, moving either the <slot> or
   the assigned node around in the tree (or across documents)
   would "break" the linkage. Now, the linkage is more permanent,
   and the only way to break it is through another call to .assign().

See [1] for the chromestatus entry, [2] for the intent to ship,
[3], [4], and [5] for the spec PRs, and [6]/[7] for the landed spec.

[1] https://chromestatus.com/feature/4979822998585344
[2] https://groups.google.com/a/chromium.org/g/blink-dev/c/6U78F3KWJ78
[3] whatwg/html#6561
[4] whatwg/html#6585
[5] whatwg/dom#966
[6] https://dom.spec.whatwg.org/#find-slotables
[7] https://html.spec.whatwg.org/#dom-slot-assign

Fixed: 1196842
Fixed: 1067153
Bug: 1067157
Change-Id: I0ee71043c23f3b49a1461296d722045f06eca540
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2824763
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#874413}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.