style: Implement shadow part forwarding (minus invalidation).

Some of the stuff, in particular inside GeckoBindings stuff should be
refactored to be less ugly and duplicate a bit less code, but the rest of the
code should be landable as is.

Some invalidation changes are already needed because we weren't matching with
the right shadow host during invalidation (which made existing ::part() tests
fail).

Pending invalidation work:

 * Making exportparts work right on the snapshots.
 * Invalidating parts from descendant hosts.

They're not very hard but I need to think how to best implement it:

 * Maybe get rid of ShadowRoot::mParts and just walk DOM descendants in the
   Shadow DOM.

 * Maybe implement a ElementHasExportPartsAttr much like HasPartAttr and use
   that to keep the list of elements.

 * Maybe invalidate :host and ::part() together in here[1]

 * Maybe something else.

Opinions?

[1]: https://searchfox.org/mozilla-central/rev/131338e5017bc0283d86fb73844407b9a2155c98/servo/components/style/invalidation/element/invalidator.rs#561

Differential Revision: https://phabricator.services.mozilla.com/D53730

components/style/invalidation/element/document_state.rs

@@ -79,7 +79,13 @@ where
                     continue;
                 }
 
-                self_invalidations.push(Invalidation::new(&dependency.selector, 0));
+                // We pass `None` as a scope, as document state selectors aren't
+                // affected by the current scope.
+                self_invalidations.push(Invalidation::new(
+                    &dependency.selector,
+                    /* scope = */ None,
+                    0,
+                ));
             }
         }