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

base/prefs/pref_notifier_impl.cc

Index: base/prefs/pref_notifier_impl.cc
diff --git a/base/prefs/pref_notifier_impl.cc b/base/prefs/pref_notifier_impl.cc
index c02a7b3f5a5bfd8d4bd5c56ce20ca699e6d10346..d6f6af616460cf50b4391568024b1089bf83e906 100644
--- a/base/prefs/pref_notifier_impl.cc
+++ b/base/prefs/pref_notifier_impl.cc
@@ -38,7 +38,7 @@ PrefNotifierImpl::~PrefNotifierImpl() {
   init_observers_.clear();
 }
 
-void PrefNotifierImpl::AddPrefObserver(const char* path,
+void PrefNotifierImpl::AddPrefObserver(const std::string& path,
                                        PrefObserver* obs) {
   // Get the pref observer list associated with the path.
   PrefObserverList* observer_list = NULL;
@@ -56,7 +56,7 @@ void PrefNotifierImpl::AddPrefObserver(const char* path,
   observer_list->AddObserver(obs);
 }
 
-void PrefNotifierImpl::RemovePrefObserver(const char* path,
+void PrefNotifierImpl::RemovePrefObserver(const std::string& path,
                                           PrefObserver* obs) {
   DCHECK(thread_checker_.CalledOnValidThread());