C++ 17 style nested namespaces (#1326)

Summary:
X-link: https://github.com/facebook/react-native/pull/38304

Pull Request resolved: https://github.com/facebook/yoga/pull/1326

For better readability

Reviewed By: christophpurrer

Differential Revision: D47384926

fbshipit-source-id: 2f60d50a185331b3624d45d1fc45f98d504b3034

java/jni/corefunctions.cpp

@@ -9,9 +9,7 @@
 #include "macros.h"
 #include "YogaJniException.h"
 
-namespace facebook {
-namespace yoga {
-namespace vanillajni {
+namespace facebook::yoga::vanillajni {
 
 namespace {
 JavaVM* globalVm = NULL;
@@ -90,6 +88,4 @@ void assertNoPendingJniExceptionIf(JNIEnv* env, bool condition) {
   throw YogaJniException();
 }
 
-} // namespace vanillajni
-} // namespace yoga
-} // namespace facebook
+} // namespace facebook::yoga::vanillajni