bottom styling isn't applied when a View is not absolute-positioned

Summary: The new CSSLayout have a wrong calculate for the getRelativePosition. So use the getLeadingPosition will get 0 instead of return undefined. Fix it with using isLeadingPosDefined.

Reviewed By: fkgozali

Differential Revision: D3640799

fbshipit-source-id: 50d3bd2ea4c0d8bf96ba34297425ba269b0535cd

Author: MengjueW

React/CSSLayout/CSSLayout.c

@@ -700,9 +700,8 @@ static void setTrailingPosition(CSSNode* node, CSSNode* child, CSSFlexDirection
 // If both left and right are defined, then use left. Otherwise return
 // +left or -right depending on which is defined.
 static float getRelativePosition(CSSNode* node, CSSFlexDirection axis) {
-  float lead = getLeadingPosition(node, axis);
-  if (!isUndefined(lead)) {
-    return lead;
+  if (isLeadingPosDefined(node, axis)) {
+    return getLeadingPosition(node, axis);
   }
   return -getTrailingPosition(node, axis);
 }