style: Improve Percentage -> LengthPercentage conversion

This doesn't change behavior because we only use them for images that
have no clamping.

Depends on D147008

Differential Revision: https://phabricator.services.mozilla.com/D147511

components/style/values/specified/image.rs

@@ -733,12 +733,12 @@ impl Gradient {
         if items.is_empty() {
             items = vec![
                 generic::GradientItem::ComplexColorStop {
-                    color: Color::transparent().into(),
-                    position: Percentage::zero().into(),
+                    color: Color::transparent(),
+                    position: LengthPercentage::zero_percent(),
                 },
                 generic::GradientItem::ComplexColorStop {
-                    color: Color::transparent().into(),
-                    position: Percentage::hundred().into(),
+                    color: Color::transparent(),
+                    position: LengthPercentage::hundred_percent(),
                 },
             ];
         } else if items.len() == 1 {

components/style/values/specified/length.rs

@@ -1328,10 +1328,9 @@ impl From<NoCalcLength> for LengthPercentage {
 impl From<Percentage> for LengthPercentage {
     #[inline]
     fn from(pc: Percentage) -> Self {
-        if pc.is_calc() {
-            // FIXME(emilio): Hard-coding the clamping mode is suspect.
+        if let Some(clamping_mode) = pc.calc_clamping_mode() {
             LengthPercentage::Calc(Box::new(CalcLengthPercentage {
-                clamping_mode: AllowedNumericType::All,
+                clamping_mode,
                 node: CalcNode::Leaf(calc::Leaf::Percentage(pc.get())),
             }))
         } else {
@@ -1364,6 +1363,12 @@ impl LengthPercentage {
         LengthPercentage::Percentage(computed::Percentage::zero())
     }
 
+    #[inline]
+    /// Returns a `100%` value.
+    pub fn hundred_percent() -> LengthPercentage {
+        LengthPercentage::Percentage(computed::Percentage::hundred())
+    }
+
     fn parse_internal<'i, 't>(
         context: &ParserContext,
         input: &mut Parser<'i, 't>,

components/style/values/specified/percentage.rs

@@ -97,6 +97,11 @@ impl Percentage {
         self.calc_clamping_mode.is_some()
     }
 
+    /// Returns the calc() clamping mode for this percentage.
+    pub fn calc_clamping_mode(&self) -> Option<AllowedNumericType> {
+        self.calc_clamping_mode
+    }
+
     /// Reverses this percentage, preserving calc-ness.
     ///
     /// For example: If it was 20%, convert it into 80%.