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/selectors/matching.rs

@@ -8,6 +8,7 @@ use crate::nth_index_cache::NthIndexCacheInner;
 use crate::parser::{AncestorHashes, Combinator, Component, LocalName};
 use crate::parser::{NonTSPseudoClass, Selector, SelectorImpl, SelectorIter, SelectorList};
 use crate::tree::Element;
+use smallvec::SmallVec;
 use std::borrow::Borrow;
 use std::iter;
 
@@ -667,7 +668,41 @@ where
 
     match *selector {
         Component::Combinator(_) => unreachable!(),
-        Component::Part(ref parts) => parts.iter().all(|part| element.is_part(part)),
+        Component::Part(ref parts) => {
+            let mut hosts = SmallVec::<[E; 4]>::new();
+
+            let mut host = match element.containing_shadow_host() {
+                Some(h) => h,
+                None => return false,
+            };
+
+            loop {
+                let outer_host = host.containing_shadow_host();
+                if outer_host.as_ref().map(|h| h.opaque()) == context.shared.current_host {
+                    break;
+                }
+                let outer_host = match outer_host {
+                    Some(h) => h,
+                    None => return false,
+                };
+                // TODO(emilio): if worth it, we could early return if
+                // host doesn't have the exportparts attribute.
+                hosts.push(host);
+                host = outer_host;
+            }
+
+            // Translate the part into the right scope.
+            parts.iter().all(|part| {
+                let mut part = part.clone();
+                for host in hosts.iter().rev() {
+                    part = match host.imported_part(&part) {
+                        Some(p) => p,
+                        None => return false,
+                    };
+                }
+                element.is_part(&part)
+            })
+        },
         Component::Slotted(ref selector) => {
             // <slots> are never flattened tree slottables.
             !element.is_html_slot_element() &&