Move Singleton and related structs to namespace base

components/sessions/ios/ios_serialized_navigation_driver.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SESSIONS_IOS_IOS_SERIALIZED_NAVIGATION_DRIVER_H_
 #define COMPONENTS_SESSIONS_IOS_IOS_SERIALIZED_NAVIGATION_DRIVER_H_
 
 #include "components/sessions/core/serialized_navigation_driver.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace sessions {
 
 // Provides an iOS implementation of SerializedNavigationDriver that is backed
 // by //ios/web classes.
 class IOSSerializedNavigationDriver
     : public SerializedNavigationDriver {
  public:
   ~IOSSerializedNavigationDriver() override;
 
   // Returns the singleton IOSSerializedNavigationDriver.  Almost all
   // callers should use SerializedNavigationDriver::Get() instead.
   static IOSSerializedNavigationDriver* GetInstance();
 
   // SerializedNavigationDriver implementation.
   int GetDefaultReferrerPolicy() const override;
   bool MapReferrerPolicyToOldValues(int referrer_policy,
                                     int* mapped_referrer_policy) const override;
   bool MapReferrerPolicyToNewValues(int referrer_policy,
                                     int* mapped_referrer_policy) const override;
   std::string GetSanitizedPageStateForPickle(
       const SerializedNavigationEntry* navigation) const override;
   void Sanitize(SerializedNavigationEntry* navigation) const override;
   std::string StripReferrerFromPageState(
       const std::string& page_state) const override;
 
  private:
   IOSSerializedNavigationDriver();
-  friend struct DefaultSingletonTraits<IOSSerializedNavigationDriver>;
+  friend struct base::DefaultSingletonTraits<IOSSerializedNavigationDriver>;
 };
 
 }  // namespace sessions
 
 #endif  // COMPONENTS_SESSIONS_IOS_IOS_SERIALIZED_NAVIGATION_DRIVER_H_