Modify ThreadChecker and NonThreadSafe so that their functionality is

base/threading/non_thread_safe.h

Index: base/threading/non_thread_safe.h
diff --git a/base/threading/non_thread_safe.h b/base/threading/non_thread_safe.h
index 868a031e2f6984258e59687daaaa08379f3a1430..68206c3195cf11a5b7e36f14fec5d17bc50ec36e 100644
--- a/base/threading/non_thread_safe.h
+++ b/base/threading/non_thread_safe.h
@@ -10,9 +10,12 @@
 
 namespace base {
 
-// A helper class used to help verify that methods of a class are
-// called from the same thread.  One can inherit from this class and use
-// CalledOnValidThread() to verify.
+class NonThreadSafeImpl;
+class NonThreadSafeDoNothing;
+
+// NonThreadSafe is a helper class used to help verify that methods of a
+// class are called from the same thread.  One can inherit from this class
+// and use CalledOnValidThread() to verify.
 //
 // This is intended to be used with classes that appear to be thread safe, but
 // aren't.  For example, a service or a singleton like the preferences system.
@@ -29,9 +32,20 @@ namespace base {
 // In Release mode, CalledOnValidThread will always return true.
 //
 #ifndef NDEBUG
-class NonThreadSafe {
+typedef NonThreadSafeImpl NonThreadSafe;
+#else
+typedef NonThreadSafeDoNothing NonThreadSafe;
+#endif  // NDEBUG
+
+// Full implementation of NonThreadSafe, for debug mode or for occasional
+// temporary use in release mode e.g. when you need to CHECK on a thread
+// bug that only occurs in the wild.
+//
+// Note: You should almost always use the NonThreadSafe typedef to get
+// the right version of the class.
+class NonThreadSafeImpl {
  public:
-  ~NonThreadSafe();
+  ~NonThreadSafeImpl();
 
   bool CalledOnValidThread() const;
 
@@ -44,11 +58,14 @@ class NonThreadSafe {
   void DetachFromThread();
 
  private:
-  ThreadChecker thread_checker_;
+  ThreadCheckerImpl thread_checker_;
 };
-#else
-// Do nothing in release mode.
-class NonThreadSafe {
+
+// Do nothing implementation of NonThreadSafe, for release mode.
+//
+// Note: You should almost always use the NonThreadSafe typedef to get
+// the right version of the class.
+class NonThreadSafeDoNothing {
  public:
   bool CalledOnValidThread() const {
     return true;
@@ -57,7 +74,6 @@ class NonThreadSafe {
  protected:
   void DetachFromThread() {}
 };
-#endif  // NDEBUG
 
 }  // namespace base