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

yoga/YGConfig.cpp

@@ -9,16 +9,14 @@
 
 using namespace facebook::yoga;
 
-namespace facebook {
-namespace yoga {
+namespace facebook::yoga {
 bool configUpdateInvalidatesLayout(YGConfigRef a, YGConfigRef b) {
   return a->getErrata() != b->getErrata() ||
       a->getEnabledExperiments() != b->getEnabledExperiments() ||
       a->getPointScaleFactor() != b->getPointScaleFactor() ||
       a->useWebDefaults() != b->useWebDefaults();
 }
-} // namespace yoga
-} // namespace facebook
+} // namespace facebook::yoga
 
 YGConfig::YGConfig(YGLogger logger) : cloneNodeCallback_{nullptr} {
   setLogger(logger);