yoga::resolveValue -> Length::resolve (#1520)

Summary:
X-link: https://github.com/facebook/react-native/pull/41939

Pull Request resolved: https://github.com/facebook/yoga/pull/1520

This code originates as `YGValueResolve`, used to compute a YGValue to a length in points, using a reference for 100%.

This moves it to `Style::Length`, so we can encapsulate parts of it (for style value functions), and make the API more cohesive now that we can do C++ style OOP with it.

Changelog: [Internal]

Reviewed By: joevilches

Differential Revision: D51796973

fbshipit-source-id: a7c359c7544f4bd2066a80d976dde67a0d16f1dd

yoga/style/StyleLength.h

@@ -71,6 +71,17 @@ class StyleLength {
     return unit_;
   }
 
+  constexpr FloatOptional resolve(float referenceLength) {
+    switch (unit_) {
+      case Unit::Point:
+        return value_;
+      case Unit::Percent:
+        return FloatOptional{value_.unwrap() * referenceLength * 0.01f};
+      default:
+        return FloatOptional{};
+    }
+  }
+
   explicit constexpr operator YGValue() const {
     return YGValue{value_.unwrap(), unscopedEnum(unit_)};
   }