Issue #1925 - Convert NS_FULL_TO_HALF_CORNER to a constexpr function.

Note: Because the new function checks types, we need to change the
fullCorner type in `nsComputedDOMStyle::GetEllipseRadii()` and
`StyleAnimationValue::ExtractComputedValue()` to `Corner` instead of the
underlying base type.

layout/style/nsComputedDOMStyle.h

@@ -158,7 +158,7 @@ private:
 #undef STYLE_STRUCT
 
   already_AddRefed<CSSValue> GetEllipseRadii(const nsStyleCorners& aRadius,
-                                             uint8_t aFullCorner);
+                                             mozilla::Corner aFullCorner);
 
   already_AddRefed<CSSValue> GetOffsetWidthFor(mozilla::Side aSide);