Bug 1980539 - Restrict NonTSPseudoClass::Heading usage to Gecko. r=emilio,firefox-style-system-reviewers

This is a fixup for D255407, which added `NonTSPseudoClass::Heading` for
Gecko but not for Servo, while still referencing it from both shared and
Servo-specific code.

Differential Revision: https://phabricator.services.mozilla.com/D259493

Author: Loirooriol

style/invalidation/element/element_wrapper.rs

@@ -225,6 +225,7 @@ where
             },
 
             // :heading should match against snapshot before element
+            #[cfg(feature = "gecko")]
             NonTSPseudoClass::Heading(ref levels) => {
                 return levels.matches_state(
                     self.snapshot()

style/servo/selector_parser.rs

@@ -384,7 +384,7 @@ impl ToCss for NonTSPseudoClass {
             Self::UserValid => ":user-valid",
             Self::Valid => ":valid",
             Self::Visited => ":visited",
-            Self::Lang(_) | Self::CustomState(_) | Self::Heading(_) => unreachable!(),
+            Self::Lang(_) | Self::CustomState(_) => unreachable!(),
         })
     }
 }
@@ -427,7 +427,7 @@ impl NonTSPseudoClass {
             Self::UserValid => ElementState::USER_VALID,
             Self::Valid => ElementState::VALID,
             Self::Visited => ElementState::VISITED,
-            Self::CustomState(_) | Self::Lang(_) | Self::Heading(_) | Self::ServoNonZeroBorder => ElementState::empty(),
+            Self::CustomState(_) | Self::Lang(_) | Self::ServoNonZeroBorder => ElementState::empty(),
         }
     }