Skip trailing position loop, if possible

There's no need to set the trailing position on left-to-right layouts
as the nodes will already have what we need (x, y, width, and height).
This means we still have an extra cost for reversed layout directions
but they are not as common as LTR ones.

src/Layout.js

@@ -909,7 +909,10 @@ var computeLayout = (function() {
         paddingAndBorderAxisMain
       );
 
-      needsMainTrailingPos = true;
+      if (mainAxis == CSS_FLEX_DIRECTION_ROW_REVERSE ||
+          mainAxis == CSS_FLEX_DIRECTION_COLUMN_REVERSE) {
+        needsMainTrailingPos = true;
+      }
     }
 
     if (!isCrossDimDefined) {
@@ -921,7 +924,10 @@ var computeLayout = (function() {
         paddingAndBorderAxisCross
       );
 
-      needsCrossTrailingPos = true;
+      if (crossAxis == CSS_FLEX_DIRECTION_ROW_REVERSE ||
+          crossAxis == CSS_FLEX_DIRECTION_COLUMN_REVERSE) {
+        needsCrossTrailingPos = true;
+      }
     }
 
     // <Loop F> Set trailing position if necessary