style: Unship line-height: -moz-block-height.

They're only used in forms.css, and only for some anonymous content, which are
not content-accessible in the first place.

The only place where this could be exposed is calling
getComputedStyle(input, "::placeholder"), so I think this should be pretty safe,
but I've added a pref just in case.

While at it, also derive the Parse implementation. Less code is better.

Differential Revision: https://phabricator.services.mozilla.com/D25118

components/style/values/generics/text.rs

@@ -61,6 +61,14 @@ impl<Value> Spacing<Value> {
     }
 }
 
+#[cfg(feature = "gecko")]
+fn line_height_moz_block_height_enabled(context: &ParserContext) -> bool {
+    use crate::gecko_bindings::structs;
+    context.in_ua_sheet() || unsafe {
+        structs::StaticPrefs_sVarCache_layout_css_line_height_moz_block_height_content_enabled
+    }
+}
+
 /// A generic value for the `line-height` property.
 #[derive(
     Animate,
@@ -73,6 +81,7 @@ impl<Value> Spacing<Value> {
     SpecifiedValueInfo,
     ToAnimatedValue,
     ToCss,
+    Parse,
 )]
 #[repr(C, u8)]
 pub enum GenericLineHeight<N, L> {
@@ -80,6 +89,7 @@ pub enum GenericLineHeight<N, L> {
     Normal,
     /// `-moz-block-height`
     #[cfg(feature = "gecko")]
+    #[parse(condition = "line_height_moz_block_height_enabled")]
     MozBlockHeight,
     /// `<number>`
     Number(N),

components/style/values/specified/text.rs

@@ -73,33 +73,6 @@ impl Parse for WordSpacing {
     }
 }
 
-impl Parse for LineHeight {
-    fn parse<'i, 't>(
-        context: &ParserContext,
-        input: &mut Parser<'i, 't>,
-    ) -> Result<Self, ParseError<'i>> {
-        if let Ok(number) = input.try(|i| NonNegativeNumber::parse(context, i)) {
-            return Ok(GenericLineHeight::Number(number));
-        }
-        if let Ok(nlp) = input.try(|i| NonNegativeLengthPercentage::parse(context, i)) {
-            return Ok(GenericLineHeight::Length(nlp));
-        }
-        let location = input.current_source_location();
-        let ident = input.expect_ident()?;
-        match ident {
-            ref ident if ident.eq_ignore_ascii_case("normal") => Ok(GenericLineHeight::Normal),
-            #[cfg(feature = "gecko")]
-            ref ident if ident.eq_ignore_ascii_case("-moz-block-height") => {
-                Ok(GenericLineHeight::MozBlockHeight)
-            },
-            ident => {
-                Err(location
-                    .new_custom_error(SelectorParseErrorKind::UnexpectedIdent(ident.clone())))
-            },
-        }
-    }
-}
-
 impl ToComputedValue for LineHeight {
     type ComputedValue = ComputedLineHeight;