Introduce CSSNodeGetInstanceCount API

Summary:
- Add CSSNodeGetInstanceCount API to get the number of native instances.
- It makes testing easy.

Reviewed By: emilsjolander

Differential Revision: D3981990

fbshipit-source-id: 98005ae1fc21d4c8802f24030fff9ffb00bd292d

CSSLayout/CSSLayout.c

@@ -129,9 +129,12 @@ computedEdgeValue(const float edges[CSSEdgeCount], const CSSEdge edge, const flo
   return defaultValue;
 }
 
+static int32_t gNodeInstanceCount = 0;
+
 CSSNodeRef CSSNodeNew() {
   const CSSNodeRef node = calloc(1, sizeof(CSSNode));
   CSS_ASSERT(node, "Could not allocate memory for node");
+  gNodeInstanceCount++;
 
   CSSNodeInit(node);
   return node;
@@ -140,6 +143,7 @@ CSSNodeRef CSSNodeNew() {
 void CSSNodeFree(const CSSNodeRef node) {
   CSSNodeListFree(node->children);
   free(node);
+  gNodeInstanceCount--;
 }
 
 void CSSNodeFreeRecursive(const CSSNodeRef root) {
@@ -151,6 +155,10 @@ void CSSNodeFreeRecursive(const CSSNodeRef root) {
   CSSNodeFree(root);
 }
 
+int32_t CSSNodeGetInstanceCount() {
+  return gNodeInstanceCount;
+}
+
 void CSSNodeInit(const CSSNodeRef node) {
   node->parent = NULL;
   node->children = CSSNodeListNew(4);