Move Singleton and related structs to namespace base

ui/gfx/win/singleton_hwnd.h

Index: ui/gfx/win/singleton_hwnd.h
diff --git a/ui/gfx/win/singleton_hwnd.h b/ui/gfx/win/singleton_hwnd.h
index ea62b3ed4b6d56be225de8f0e8f9ee7085cec54b..fe2e2986324c7ed897b851ae8786a4071f04af75 100644
--- a/ui/gfx/win/singleton_hwnd.h
+++ b/ui/gfx/win/singleton_hwnd.h
@@ -12,7 +12,9 @@
 #include "ui/gfx/gfx_export.h"
 #include "ui/gfx/win/window_impl.h"
 
+namespace base {
 template<typename T> struct DefaultSingletonTraits;
+}
 
 namespace gfx {
 
@@ -34,7 +36,7 @@ class GFX_EXPORT SingletonHwnd : public WindowImpl {
 
  private:
   friend class SingletonHwndObserver;
-  friend struct DefaultSingletonTraits<SingletonHwnd>;
+  friend struct base::DefaultSingletonTraits<SingletonHwnd>;
 
   SingletonHwnd();
   ~SingletonHwnd() override;