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

base/prefs/pref_member.cc

Index: base/prefs/pref_member.cc
diff --git a/base/prefs/pref_member.cc b/base/prefs/pref_member.cc
index 4fa616f4fb612bb8bc3c6338d39a7f60a3033700..024c693438163de45f9b3ba6872aa9899e8b359a 100644
--- a/base/prefs/pref_member.cc
+++ b/base/prefs/pref_member.cc
@@ -25,16 +25,14 @@ PrefMemberBase::~PrefMemberBase() {
   Destroy();
 }
 
-void PrefMemberBase::Init(const char* pref_name,
+void PrefMemberBase::Init(const std::string& pref_name,
                           PrefService* prefs,
                           const NamedChangeCallback& observer) {
   observer_ = observer;
   Init(pref_name, prefs);
 }
 
-void PrefMemberBase::Init(const char* pref_name,
-                          PrefService* prefs) {
-  DCHECK(pref_name);
+void PrefMemberBase::Init(const std::string& pref_name, PrefService* prefs) {
   DCHECK(prefs);
   DCHECK(pref_name_.empty());  // Check that Init is only called once.
   prefs_ = prefs;