Introduce CSSAssertSetFailFunc and CSSAsserFail to throw managed exception

Summary:
- Define CSS_ASSERT_FAIL_ENABLED for P/Invoke (Visual Studio project already has it)
- Pass managed delegate pointer to unmanaged side via P/Invoke.
- CSSAssertFail will call the managed delegate when assert failed.
- The delegate will throw managed exception.

Reviewed By: emilsjolander

Differential Revision: D3982084

fbshipit-source-id: 058a87c10ca89238362be4d8759cc00dd0c9b376

csharp/Facebook.CSSLayout/Native.cs

@@ -20,6 +20,9 @@ namespace Facebook.CSSLayout
         private const string DllName = "CSSLayout";
 #endif
 
+        [DllImport(DllName)]
+        public static extern void CSSAssertSetFailFunc(CSSAssert.FailFunc func);
+
         [DllImport(DllName)]
         public static extern IntPtr CSSNodeNew();