Bug 1374233 - Part 8: Implement ToAnimatedValue for BorderCornerRadius.

BorderCornerRadius should always be non-negative, so we can implemennt
ToAnimatedValue for it directly, for properties:
1. border-{*}-radius
2. -moz-outline-{*}-radius

MozReview-Commit-ID: HEbeHz9Hfkd

components/style/properties/longhand/border.mako.rs

@@ -62,7 +62,7 @@ ${helpers.gecko_keyword_conversion(Keyword('border-style',
                               spec="https://drafts.csswg.org/css-backgrounds/#border-%s-radius" % corner,
                               boxed=True,
                               flags="APPLIES_TO_FIRST_LETTER",
-                              animation_value_type="ComputedValue")}
+                              animation_value_type="BorderCornerRadius")}
 % endfor
 
 /// -moz-border-*-colors: color, string, enum, none, inherit/initial