Avoid extra work when justifyContent is FLEX_START

Remaining space and between space only need to be updated when
justifyContent is not FLEX_START.

src/java/src/com/facebook/csslayout/LayoutEngine.java

@@ -650,7 +650,7 @@ public class LayoutEngine {
   
       // We use justifyContent to figure out how to allocate the remaining
       // space available
-      } else {
+      } else if (node.style.justifyContent != CSSJustify.FLEX_START) {
         CSSJustify justifyContent = node.style.justifyContent;
         if (justifyContent == CSSJustify.CENTER) {
           leadingMainDim = remainingMainDim / 2;