Move Singleton and related structs to namespace base

base/threading/thread_id_name_manager.h

Index: base/threading/thread_id_name_manager.h
diff --git a/base/threading/thread_id_name_manager.h b/base/threading/thread_id_name_manager.h
index 927d25fe1e8eda060393eba71d1af29e531bf56c..1ba7e13e57bf487b8b978fcf481e9ecb0121b116 100644
--- a/base/threading/thread_id_name_manager.h
+++ b/base/threading/thread_id_name_manager.h
@@ -13,10 +13,11 @@
 #include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
 
-template <typename T> struct DefaultSingletonTraits;
-
 namespace base {
 
+template <typename T>
+struct DefaultSingletonTraits;
+
 class BASE_EXPORT ThreadIdNameManager {
  public:
   static ThreadIdNameManager* GetInstance();