YGNode::getChildren() should return const ref

Summary:
It's wasteful to do it by value. I'm fairly sure this is
safe, especially because
fbd332dee8 (diff-ade2a4bbd6582e2898cbd9e0fa142ab5R215)
shows that we did access by reference before.

Reviewed By: priteshrnandgaonkar, davidaurelio

Differential Revision: D8822697

fbshipit-source-id: 791bcf0fa37453f67795af727c85c8adce3b0f69

yoga/YGNode.h

@@ -122,14 +122,10 @@ struct YGNode {
     return getOwner();
   }
   
-  YGVector getChildren() const {
+  const YGVector& getChildren() const {
     return children_;
   }
   
-  uint32_t getChildrenCount() const {
-    return static_cast<uint32_t>(children_.size());
-  }
-  
   YGNodeRef getChild(uint32_t index) const {
     return children_.at(index);
   }

yoga/Yoga.cpp

@@ -264,7 +264,7 @@ static YGNodeRef YGNodeDeepClone(YGNodeRef oldNode) {
   YGVector vec = YGVector();
   vec.reserve(oldNode->getChildren().size());
   YGNodeRef childNode = nullptr;
-  for (auto& item : oldNode->getChildren()) {
+  for (auto* item : oldNode->getChildren()) {
     childNode = YGNodeDeepClone(item);
     childNode->setOwner(node);
     vec.push_back(childNode);
@@ -301,8 +301,8 @@ static void YGConfigFreeRecursive(const YGNodeRef root) {
     delete root->getConfig();
   }
   // Delete configs recursively for childrens
-  for (uint32_t i = 0; i < root->getChildrenCount(); ++i) {
-    YGConfigFreeRecursive(root->getChild(i));
+  for (auto* child : root->getChildren()) {
+    YGConfigFreeRecursive(child);
   }
 }
 
@@ -1868,7 +1868,7 @@ static YGCollectFlexItemsRowValues YGCalculateCollectFlexItemsRowValues(
 
   // Add items to the current line until it's full or we run out of items.
   uint32_t endOfLineIndex = startOfLineIndex;
-  for (; endOfLineIndex < node->getChildrenCount(); endOfLineIndex++) {
+  for (; endOfLineIndex < node->getChildren().size(); endOfLineIndex++) {
     const YGNodeRef child = node->getChild(endOfLineIndex);
     if (child->getStyle().display == YGDisplayNone ||
         child->getStyle().positionType == YGPositionTypeAbsolute) {