Modify ThreadChecker and NonThreadSafe so that their functionality is

base/threading/thread_checker.h

Index: base/threading/thread_checker.h
diff --git a/base/threading/thread_checker.h b/base/threading/thread_checker.h
index 712b5b56f03efeb429dd9231bfc8807a99c0dc32..b04bb6f7b4f65bb157f9f7e73c3ce3a4d0485f30 100644
--- a/base/threading/thread_checker.h
+++ b/base/threading/thread_checker.h
@@ -6,23 +6,24 @@
 #define BASE_THREADING_THREAD_CHECKER_H_
 #pragma once
 
-#ifndef NDEBUG
 #include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
-#endif // NDEBUG
 
 namespace base {
 
+class ThreadCheckerImpl;
+class ThreadCheckerDoNothing;
+
 // Before using this class, please consider using NonThreadSafe as it
 // makes it much easier to determine the nature of your class.
 //
-// A helper class used to help verify that some methods of a class are
-// called from the same thread.  One can inherit from this class and use
-// CalledOnValidThread() to verify.
+// ThreadChecker is a helper class used to help verify that some methods of a
+// class are called from the same thread.  One can inherit from this class and
+// use CalledOnValidThread() to verify.
 //
-// Inheriting from class indicates that one must be careful when using the class
-// with multiple threads. However, it is up to the class document to indicate
-// how it can be used with threads.
+// Inheriting from class indicates that one must be careful when using the
+// class with multiple threads. However, it is up to the class document to
+// indicate how it can be used with threads.
 //
 // Example:
 // class MyClass : public ThreadChecker {
@@ -34,12 +35,22 @@ namespace base {
 // }
 //
 // In Release mode, CalledOnValidThread will always return true.
-//
 #ifndef NDEBUG
-class ThreadChecker {
+typedef ThreadCheckerImpl ThreadChecker;
+#else
+typedef ThreadCheckerDoNothing ThreadChecker;
+#endif  // NDEBUG
+
+// Real implementation, for use in debug mode, or for temporary use
+// in release mode (e.g. to CHECK on a threading issue seen only in
+// the wild).
+//
+// Note: You should almost always use the ThreadChecker typedef to get the
+// right version.
+class ThreadCheckerImpl {
  public:
-  ThreadChecker();
-  ~ThreadChecker();
+  ThreadCheckerImpl();
+  ~ThreadCheckerImpl();
 
   bool CalledOnValidThread() const;
 
@@ -56,9 +67,12 @@ class ThreadChecker {
   // It's guarded by |lock_|.
   mutable PlatformThreadId valid_thread_id_;
 };
-#else
-// Do nothing in release mode.
-class ThreadChecker {
+
+// Do nothing implementation, for use in release mode.
+//
+// Note: You should almost always use the ThreadChecker typedef to get the
+// right version.
+class ThreadCheckerDoNothing {
  public:
   bool CalledOnValidThread() const {
     return true;
@@ -66,7 +80,6 @@ class ThreadChecker {
 
   void DetachFromThread() {}
 };
-#endif  // NDEBUG
 
 }  // namespace base