style: Flip boolean half way for path interpolation.

According to the new svg 2 spec update (#543), we flip the flag half way for
path interpolation.

Differential Revision: https://phabricator.services.mozilla.com/D6192

components/style/values/specified/svg_path.rs

@@ -13,7 +13,7 @@ use std::slice;
 use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};
 use style_traits::values::SequenceWriter;
 use values::CSSFloat;
-use values::animated::{Animate, Procedure};
+use values::animated::{Animate, Procedure, ToAnimatedZero};
 use values::distance::{ComputeSquaredDistance, SquaredDistance};
 
 /// The SVG path data.
@@ -198,9 +198,7 @@ pub enum PathCommand {
         rx: CSSFloat,
         ry: CSSFloat,
         angle: CSSFloat,
-        #[animation(constant)]
         large_arc_flag: ArcFlag,
-        #[animation(constant)]
         sweep_flag: ArcFlag,
         point: CoordPair,
         absolute: IsAbsolute,
@@ -538,6 +536,34 @@ impl ToCss for ArcFlag {
     }
 }
 
+impl Animate for ArcFlag {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        (self.0 as i32)
+            .animate(&(other.0 as i32), procedure)
+            .map(|v| ArcFlag(v > 0))
+    }
+}
+
+impl ComputeSquaredDistance for ArcFlag {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        (self.0 as i32).compute_squared_distance(&(other.0 as i32))
+    }
+}
+
+impl ToAnimatedZero for ArcFlag {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        // The 2 ArcFlags in EllipticalArc determine which one of the 4 different arcs will be
+        // used. (i.e. From 4 combinations). In other words, if we change the flag, we get a
+        // different arc. Therefore, we return *self.
+        // https://svgwg.org/svg2-draft/paths.html#PathDataEllipticalArcCommands
+        Ok(*self)
+    }
+}
+
+
 /// SVG Path parser.
 struct PathParser<'a> {
     chars: Peekable<Cloned<slice::Iter<'a, u8>>>,