Move Singleton and related structs to namespace base

components/web_cache/browser/web_cache_manager.h

Index: components/web_cache/browser/web_cache_manager.h
diff --git a/components/web_cache/browser/web_cache_manager.h b/components/web_cache/browser/web_cache_manager.h
index 0d2d3ffb644a6f05234b4e54a06a6390b3007e52..230d6fd4930251dce72f45a3b0a9a3a2d165bed9 100644
--- a/components/web_cache/browser/web_cache_manager.h
+++ b/components/web_cache/browser/web_cache_manager.h
@@ -21,8 +21,11 @@
 #include "content/public/browser/notification_registrar.h"
 #include "third_party/WebKit/public/web/WebCache.h"
 
+namespace base {
 template<typename Type>
 struct DefaultSingletonTraits;
+}  // namespace base
+
 class PrefRegistrySimple;
 
 namespace web_cache {
@@ -124,7 +127,7 @@ class WebCacheManager : public content::NotificationObserver {
 
   // This class is a singleton.  Do not instantiate directly.
   WebCacheManager();
-  friend struct DefaultSingletonTraits<WebCacheManager>;
+  friend struct base::DefaultSingletonTraits<WebCacheManager>;
 
   ~WebCacheManager() override;