Describe .nc files as "No Compile Test" suites and link doc.

base/callback_unittest.nc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This is a "No Compile Test" suite.
+// http://dev.chromium.org/developers/testing/no-compile-tests
+
 #include "base/callback.h"
 
 namespace base {
 
 class Parent {
 };
 
 class Child : Parent {
 };
 
@@ skipping 26 matching lines @@
 // See explanation for NCTEST_CONSTRUCTION_FROM_SUBTYPE
 void WontCompile() {
   Callback<Parent(void)> cb_a;
   Callback<Child(void)> cb_b;
   cb_a = cb_b;
 }
 
 #endif
 
 }  // namespace base