[css] Switch part attribute to a DOMTokenList

Adds tests for manipulating it via the attribute.

Renames PartNames() to Part() to match with part(), the API wrapper.

This uses the same IDL as classList.

Bug: 805271
Change-Id: I9715df48478510029f5d985124e46111c41cffc3
Reviewed-on: https://chromium-review.googlesource.com/c/1333014
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611570}
13 files changed