Allow MeasureFunc to be set to NULL no matter how many children a node has.

Reviewed By: emilsjolander

Differential Revision: D4148727

fbshipit-source-id: 79a0f3ef1bf7b1dce9a14de96f870e35c042b78b

React/CSSLayout/CSSLayout.c

@@ -259,8 +259,13 @@ static void _CSSNodeMarkDirty(const CSSNodeRef node) {
 }
 
 void CSSNodeSetMeasureFunc(const CSSNodeRef node, CSSMeasureFunc measureFunc) {
-  CSS_ASSERT(CSSNodeChildCount(node) == 0, "Cannot set measure function: Nodes with measure functions cannot have children.");
-  node->measure = measureFunc;
+  // You can always NULLify the measure function of a node.
+  if (measureFunc == NULL) {
+    node->measure = NULL;
+  } else {
+    CSS_ASSERT(CSSNodeChildCount(node) == 0, "Cannot set measure function: Nodes with measure functions cannot have children.");
+    node->measure = measureFunc;
+  }
 }
 
 CSSMeasureFunc CSSNodeGetMeasureFunc(const CSSNodeRef node) {