style: Share CascadeData instances across ShadowRoots

This should be both a memory and speed win for pages using a lot of
Shadow DOM.

In order to make the cache properly work we need to start keying media query
results on the actual StyleSheetContents, as that's what we share on Gecko, but
that should all be fine.

Differential Revision: https://phabricator.services.mozilla.com/D107266

components/style/invalidation/element/invalidation_map.rs

@@ -185,7 +185,7 @@ pub struct DocumentStateDependency {
 /// In particular, we want to lookup as few things as possible to get the fewer
 /// selectors the better, so this looks up by id, class, or looks at the list of
 /// state/other attribute affecting selectors.
-#[derive(Debug, MallocSizeOf)]
+#[derive(Clone, Debug, MallocSizeOf)]
 pub struct InvalidationMap {
     /// A map from a given class name to all the selectors with that class
     /// selector.