Don't copy children in YGNodeComputeFlexBasisForChildren (facebook#919)

Summary:
No need for a copy here.
Pull Request resolved: facebook#919

Differential Revision: D16701461

Pulled By: davidaurelio

fbshipit-source-id: 3a90adbb2b5c43d5aefe693a8525aa3a37e53b3d

Author: Adlai-Holler

yoga/Yoga.cpp

@@ -1850,7 +1850,7 @@ static float YGNodeComputeFlexBasisForChildren(
     const uint32_t generationCount) {
   float totalOuterFlexBasis = 0.0f;
   YGNodeRef singleFlexChild = nullptr;
-  YGVector children = node->getChildren();
+  const YGVector &children = node->getChildren();
   YGMeasureMode measureModeMainDim =
       YGFlexDirectionIsRow(mainAxis) ? widthMeasureMode : heightMeasureMode;
   // If there is only one child with flexGrow + flexShrink it means we can set