Bug 1557403 - Handle default angle in radians. r=me

We were not copying the angle in this case for some reason, and I overlooked
this.

layout/style/test/property_database.js

@@ -107,6 +107,7 @@ var validGradientAndElementValues = [
   "linear-gradient(#ffff00, #ef3, rgba(10, 20, 30, 0.4))",
   "linear-gradient(rgba(10, 20, 30, 0.4), #ffff00, #ef3)",
   "linear-gradient(red, green calc(50% + 20px), blue)",
+  "linear-gradient(180deg, red, blue)",
 
   "linear-gradient(to top, red, blue)",
   "linear-gradient(to bottom, red, blue)",

servo/components/style/gecko/conversions.rs

@@ -389,6 +389,7 @@ impl nsStyleImage {
                 let angle = Angle::from_gecko_style_coord(&gecko_gradient.mAngle);
                 let line_direction = match (angle, horizontal_style, vertical_style) {
                     (Some(a), None, None) => LineDirection::Angle(a),
+                    (None, None, None) => LineDirection::Angle(Angle::from_radians(PI)),
                     (None, Some(horizontal), Some(vertical)) => {
                         line_direction(horizontal, vertical)
                     },