Auto merge of #18499 - heycam:rule-cache, r=emilio

Rule cache

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This adds a TLS-based cache reset styles structs keyed off rule nodes.  Reviewed in https://bugzilla.mozilla.org/show_bug.cgi?id=1367635 by me and Emilio.

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- [ ] These changes fix #__ (github issue number if applicable).

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components/style/values/computed/length.rs

@@ -866,6 +866,10 @@ impl ToComputedValue for specified::MozLength {
                 MozLength::LengthOrPercentageOrAuto(lopoa.to_computed_value(context))
             }
             specified::MozLength::ExtremumLength(ref ext) => {
+                debug_assert!(context.for_non_inherited_property.is_some(),
+                              "should check whether we're a non-inherited property");
+                context.rule_cache_conditions.borrow_mut()
+                    .set_writing_mode_dependency(context.builder.writing_mode);
                 MozLength::ExtremumLength(ext.clone())
             }
         }