Add YGConfigGetInstanceCount

Summary:
- depends on #496
- For memory leak unit test
- Expose the API for C#
Closes https://github.com/facebook/yoga/pull/497

Reviewed By: emilsjolander

Differential Revision: D4796190

Pulled By: splhack

fbshipit-source-id: 99e4e78e8dfb3d459cf6cd7103ab252c3748e5a6

yoga/Yoga.h

@@ -228,6 +228,7 @@ WIN_EXPORT void YGConfigSetPointScaleFactor(const YGConfigRef config, const floa
 // YGConfig
 WIN_EXPORT YGConfigRef YGConfigNew(void);
 WIN_EXPORT void YGConfigFree(const YGConfigRef config);
+WIN_EXPORT int32_t YGConfigGetInstanceCount(void);
 
 WIN_EXPORT void YGConfigSetExperimentalFeatureEnabled(const YGConfigRef config,
                                                       const YGExperimentalFeature feature,