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Add PartNames which represents a set of names and handles applying
part name maps.

Plumb this into StyleResolver to handle part name forwarding while
ascending through shadow hosts.

Add chrome-only layout tests since the spec is not final.

Bug: 805271
Change-Id: Iddd79aafee6ce79fd3fefc7e31dc81f083b880de
Reviewed-on: https://chromium-review.googlesource.com/997317
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553988}