Fabric: Resonable default values for LayoutConstraints members

Summary: Trivial.

Reviewed By: mdvacca

Differential Revision: D7857046

fbshipit-source-id: 88400f1831bf66ccd3d880ddf1de3df27287b5be

ReactCommon/fabric/core/layout/LayoutConstraints.h

@@ -17,8 +17,8 @@ namespace react {
  * Unified layout constraints for measuring.
  */
 struct LayoutConstraints {
-  Size minimumSize;
-  Size maximumSize;
+  Size minimumSize {0, 0};
+  Size maximumSize {kFloatUndefined, kFloatUndefined};
   LayoutDirection layoutDirection;
 };