Change preference APIs to take std::string instead of const char*.

base/prefs/pref_change_registrar.h

Index: base/prefs/pref_change_registrar.h
diff --git a/base/prefs/pref_change_registrar.h b/base/prefs/pref_change_registrar.h
index 70c22fe3f43a7770da949382fa6d65a4f8afd54d..acf0a684962e0b6df9e89c02f0b7b6fa3318b328 100644
--- a/base/prefs/pref_change_registrar.h
+++ b/base/prefs/pref_change_registrar.h
@@ -40,11 +40,11 @@ class BASE_PREFS_EXPORT PrefChangeRegistrar : public PrefObserver {
   // the preference that is changing as its parameter.
   //
   // Only one observer may be registered per path.
-  void Add(const char* path, const base::Closure& obs);
-  void Add(const char* path, const NamedChangeCallback& obs);
+  void Add(const std::string& path, const base::Closure& obs);
+  void Add(const std::string& path, const NamedChangeCallback& obs);
 
   // Removes the pref observer registered for |path|.
-  void Remove(const char* path);
+  void Remove(const std::string& path);
 
   // Removes all observers that have been previously added with a call to Add.
   void RemoveAll();