servo: Merge #14922 - Don't inherit all properties for eagerly computed pseudos if there are no matching rules (from heycam:pseudo-inherit); r=emilio

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When we resolve style for an eagerly computed pseudo-element, such as an anonymous box, if there are no matching rules we currently just clone the parent's style.  We should only do that if `inherit_all` is true, otherwise we should inherit only the inherited properties from the parent.

I was going to use `.unwrap_or_default()` on the result of looking up `precomputed_pseudo_element_decls`, but that didn't seem to work since that map lookup returns a reference and not a `Vec` itself.

r? @emilio

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1329121

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Source-Repo: https://github.com/servo/servo
Source-Revision: ebd4a8b3ee1aab910ef714420a814912fec31e30

servo/components/script_layout_interface/wrapper_traits.rs

@@ -394,7 +394,7 @@ pub trait ThreadSafeLayoutElement: Clone + Copy + Sized + Debug +
                                     &context.default_computed_values,
                                     false);
                             data.styles_mut().pseudos
-                                .insert(style_pseudo.clone(), new_style.unwrap());
+                                .insert(style_pseudo.clone(), new_style);
                         }
                     }
                     PseudoElementCascadeType::Lazy => {