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

csharp/Facebook.CSSLayout/Native.cs

@@ -29,6 +29,9 @@ namespace Facebook.CSSLayout
         [DllImport(DllName)]
         public static extern void CSSNodeFree(IntPtr cssNode);
 
+        [DllImport(DllName)]
+        public static extern int CSSNodeGetInstanceCount();
+
         [DllImport(DllName)]
         public static extern void CSSNodeInsertChild(IntPtr node, IntPtr child, uint index);