Enabling round-to-pixel Yoga feature for RN (iOS)

Summary:
This change enables built-in Yoga mechanism which rounds producing layout metrics to closest "pixel" values.
See previous diff for more context.

Reviewed By: fkgozali

Differential Revision: D6889762

fbshipit-source-id: bc2eea44704db4b377e2e14fab9f67be8c935719

Author: shergin

RNTester/RNTesterIntegrationTests/ReferenceImages/RNTester-js-RNTesterApp.ios/testTabBarExample_1-iOS10@2x.png

@@ -50,8 +50,7 @@ + (YGConfigRef)yogaConfig
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
     yogaConfig = YGConfigNew();
-    // Turnig off pixel rounding.
-    YGConfigSetPointScaleFactor(yogaConfig, 0.0);
+    YGConfigSetPointScaleFactor(yogaConfig, RCTScreenScale());
     YGConfigSetUseLegacyStretchBehaviour(yogaConfig, true);
   });
   return yogaConfig;