style: Percentage values of translate are serialized as percent for computed values.

Basically, we rewrite the type of generics::transform::Translate and its
ToCss to match the spec. Besides, we always serialize Translate by servo,
so we could drop a lot of duplicated code.

Differential Revision: https://phabricator.services.mozilla.com/D11206

components/style/values/specified/length.rs

@@ -14,6 +14,7 @@ use crate::values::computed::{self, CSSPixelLength, Context, ExtremumLength};
 use crate::values::generics::length::{
     MaxLength as GenericMaxLength, MozLength as GenericMozLength,
 };
+use values::generics::transform::IsZeroLength;
 use crate::values::generics::NonNegative;
 use crate::values::specified::calc::CalcNode;
 use crate::values::{Auto, CSSFloat, Either, IsAuto, Normal};
@@ -99,6 +100,16 @@ impl FontBaseSize {
 }
 
 impl FontRelativeLength {
+    /// Return true if this is a zero value.
+    fn is_zero(&self) -> bool {
+        match *self {
+            FontRelativeLength::Em(v) |
+            FontRelativeLength::Ex(v) |
+            FontRelativeLength::Ch(v) |
+            FontRelativeLength::Rem(v) => v == 0.0,
+        }
+    }
+
     /// Computes the font-relative length.
     pub fn to_computed_value(&self, context: &Context, base_size: FontBaseSize) -> CSSPixelLength {
         use std::f32;
@@ -232,6 +243,16 @@ pub enum ViewportPercentageLength {
 }
 
 impl ViewportPercentageLength {
+    /// Return true if this is a zero value.
+    fn is_zero(&self) -> bool {
+        match *self {
+            ViewportPercentageLength::Vw(v) |
+            ViewportPercentageLength::Vh(v) |
+            ViewportPercentageLength::Vmin(v) |
+            ViewportPercentageLength::Vmax(v) => v == 0.0,
+        }
+    }
+
     /// Computes the given viewport-relative length for the given viewport size.
     pub fn to_computed_value(&self, viewport_size: Size2D<Au>) -> CSSPixelLength {
         let (factor, length) = match *self {
@@ -487,6 +508,18 @@ impl NoCalcLength {
 
 impl SpecifiedValueInfo for NoCalcLength {}
 
+impl IsZeroLength for NoCalcLength {
+    #[inline]
+    fn is_zero_length(&self) -> bool {
+        match *self {
+            NoCalcLength::Absolute(v) => v.is_zero(),
+            NoCalcLength::FontRelative(v) => v.is_zero(),
+            NoCalcLength::ViewportPercentage(v) => v.is_zero(),
+            NoCalcLength::ServoCharacterWidth(v) => v.0 == 0,
+        }
+    }
+}
+
 /// An extension to `NoCalcLength` to parse `calc` expressions.
 /// This is commonly used for the `<length>` values.
 ///
@@ -841,6 +874,17 @@ impl LengthOrPercentage {
     }
 }
 
+impl IsZeroLength for LengthOrPercentage {
+    #[inline]
+    fn is_zero_length(&self) -> bool {
+        match *self {
+            LengthOrPercentage::Length(l) => l.is_zero_length(),
+            LengthOrPercentage::Percentage(p) => p.0 == 0.0,
+            LengthOrPercentage::Calc(_) => false,
+        }
+    }
+}
+
 /// Either a `<length>`, a `<percentage>`, or the `auto` keyword.
 #[allow(missing_docs)]
 #[derive(Clone, Debug, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToCss)]