https://drafts.csswg.org/css-shadow-parts/

Tests cover the following:
- Simple - Part in selected host is styled
- Host Stylesheet - Part in selected host is not styled by ::part in a stylesheet inside the host
- Different Host - Part is not styled when host is not selected
- Inner Host
 - Part in outer host is styled by document style sheet
 - Part in inner host is not styled by document style sheet
- Compound Positive - Compound selector for host works
- Compound Negative - Non-matching compound selector for host does not style
- All Hosts
 - ::part with host selector styles in first host
 - ::part with host selector styles in second host
- Chaining Negative - ::part cannot be chained to reach elements in the inner host

Bug: 805271
Change-Id: I8c16ae09547bd833dbcc1eb0afb8be91a40c0a8e
Reviewed-on: https://chromium-review.googlesource.com/983833
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548349}