Eliminate YGFloatOptional::getValue()

Summary:
@public

Replace `YGFloatOptional::getValue()` with `YGFloatOptional::unwrap()`.

`YGFloatOptional::getValue()` has the unfortunate property of calling `std::exit` if the wrapped value is undefined.

Here, we eliminate the method, and just call `.unwrap()` everywhere.

Reviewed By: shergin

Differential Revision: D13439608

fbshipit-source-id: 5ae82b170537d0a10c301412567a7a66fd50bab4

yoga/YGNodePrint.cpp

@@ -43,7 +43,7 @@ static void appendFloatOptionalIfDefined(
     const string key,
     const YGFloatOptional num) {
   if (!num.isUndefined()) {
-    appendFormatedString(base, "%s: %g; ", key.c_str(), num.getValue());
+    appendFormatedString(base, "%s: %g; ", key.c_str(), num.unwrap());
   }
 }