Auto merge of #17197 - bholley:nonzero_arc, r=Manishearth

Force the NonZero optimization for servo_arc and StrongRuleNode

https://bugzilla.mozilla.org/show_bug.cgi?id=1370711

Also addressing followup feedback from https://bugzilla.mozilla.org/show_bug.cgi?id=1370107

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components/style/properties/properties.mako.rs

@@ -2193,7 +2193,7 @@ pub fn get_writing_mode(inheritedbox_style: &style_structs::InheritedBox) -> Wri
 }
 
 /// A reference to a style struct of the parent, or our own style struct.
-pub enum StyleStructRef<'a, T: 'a> {
+pub enum StyleStructRef<'a, T: 'static> {
     /// A borrowed struct from the parent, for example, for inheriting style.
     Borrowed(&'a Arc<T>),
     /// An owned struct, that we've already mutated.