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

base/prefs/pref_change_registrar.cc

Index: base/prefs/pref_change_registrar.cc
diff --git a/base/prefs/pref_change_registrar.cc b/base/prefs/pref_change_registrar.cc
index 28ac37403435f7428670cc032f6f603e7f6ecda6..d35416b042a7cb71cec400faff033365fae4425a 100644
--- a/base/prefs/pref_change_registrar.cc
+++ b/base/prefs/pref_change_registrar.cc
@@ -23,12 +23,12 @@ void PrefChangeRegistrar::Init(PrefService* service) {
   service_ = service;
 }
 
-void PrefChangeRegistrar::Add(const char* path,
+void PrefChangeRegistrar::Add(const std::string& path,
                               const base::Closure& obs) {
   Add(path, base::Bind(&PrefChangeRegistrar::InvokeUnnamedCallback, obs));
 }
 
-void PrefChangeRegistrar::Add(const char* path,
+void PrefChangeRegistrar::Add(const std::string& path,
                               const NamedChangeCallback& obs) {
   if (!service_) {
     NOTREACHED();
@@ -40,7 +40,7 @@ void PrefChangeRegistrar::Add(const char* path,
   observers_[path] = obs;
 }
 
-void PrefChangeRegistrar::Remove(const char* path) {
+void PrefChangeRegistrar::Remove(const std::string& path) {
   DCHECK(IsObserved(path));
 
   observers_.erase(path);