style: Add experimental support for "e", "pi", and various trigonometric functions in calc()

I'll add some tests before enabling. Also, WebKit folks (who have
implemented cos() / tan() / sin()) said they will upstream their tests
to WPT, so I'll extend those with the inverse functions before landing
as well.

Differential Revision: https://phabricator.services.mozilla.com/D124990

components/style/values/specified/angle.rs

@@ -130,6 +130,15 @@ impl Angle {
         }
     }
 
+    /// Creates an angle with the given value in radians.
+    #[inline]
+    pub fn from_radians(value: CSSFloat) -> Self {
+        Angle {
+            value: AngleDimension::Rad(value),
+            was_calc: false,
+        }
+    }
+
     /// Return `0deg`.
     pub fn zero() -> Self {
         Self::from_degrees(0.0, false)
@@ -141,6 +150,13 @@ impl Angle {
         self.value.degrees()
     }
 
+    /// Returns the value of the angle in radians.
+    #[inline]
+    pub fn radians(&self) -> CSSFloat {
+        const RAD_PER_DEG: f32 = PI / 180.0;
+        self.value.degrees() * RAD_PER_DEG
+    }
+
     /// Whether this specified angle came from a `calc()` expression.
     #[inline]
     pub fn was_calc(&self) -> bool {