For unit tests, track additions to AtExitManager and warn.

base/at_exit.h

Index: base/at_exit.h
diff --git a/base/at_exit.h b/base/at_exit.h
index 613c9fe7d1729f8a9590e50abc3f8547c0676bb8..a63acc8b5f321c93dd7c9b4e3cbc2a95c7ee6f48 100644
--- a/base/at_exit.h
+++ b/base/at_exit.h
@@ -15,6 +15,8 @@
 
 namespace base {
 
+class TestWatchAtExitManager;
+
 // This class provides a facility similar to the CRT atexit(), except that
 // we control when the callbacks are executed. Under Windows for a DLL they
 // happen at a really bad time and under the loader lock. This facility is
@@ -58,6 +60,8 @@ class BASE_EXPORT AtExitManager {
   explicit AtExitManager(bool shadow);
 
  private:
+  friend class TestWatchAtExitManager;
+
   base::Lock lock_;
   std::stack<base::Closure> stack_;
   AtExitManager* next_manager_;  // Stack of managers to allow shadowing.