Move Singleton and related structs to namespace base

components/sessions/ios/ios_serialized_navigation_driver.h

Index: components/sessions/ios/ios_serialized_navigation_driver.h
diff --git a/components/sessions/ios/ios_serialized_navigation_driver.h b/components/sessions/ios/ios_serialized_navigation_driver.h
index af7597347820077ba580f03996e603397e8287d3..7982ad3145e794307c2f5100a09e04e9f01e9666 100644
--- a/components/sessions/ios/ios_serialized_navigation_driver.h
+++ b/components/sessions/ios/ios_serialized_navigation_driver.h
@@ -7,7 +7,9 @@
 
 #include "components/sessions/core/serialized_navigation_driver.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace sessions {
 
@@ -36,7 +38,7 @@ class IOSSerializedNavigationDriver
 
  private:
   IOSSerializedNavigationDriver();
-  friend struct DefaultSingletonTraits<IOSSerializedNavigationDriver>;
+  friend struct base::DefaultSingletonTraits<IOSSerializedNavigationDriver>;
 };
 
 }  // namespace sessions