CSS: Parse partmap attribute into map in Element

Adds a NamesMap class to represent a map of part names to ordered-set of partnames.

Add partmap attribute to HTML attributes.

Adds methods on Element and ElementRareData to set/get the parsed part name map.

Bug: 805271

Change-Id: Ib17a00873cd1598b88f5e120dd96e63580390443
Reviewed-on: https://chromium-review.googlesource.com/994896
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553034}
11 files changed