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

base/prefs/scoped_user_pref_update.cc

Index: base/prefs/scoped_user_pref_update.cc
diff --git a/base/prefs/scoped_user_pref_update.cc b/base/prefs/scoped_user_pref_update.cc
index 7871c75ce3783ab84156d188fd58b087927e2cfd..364d42e9f6d6db41fa40883601ffd63dfc7cf5ef 100644
--- a/base/prefs/scoped_user_pref_update.cc
+++ b/base/prefs/scoped_user_pref_update.cc
@@ -11,10 +11,8 @@
 namespace subtle {
 
 ScopedUserPrefUpdateBase::ScopedUserPrefUpdateBase(PrefService* service,
-                                                   const char* path)
-    : service_(service),
-      path_(path),
-      value_(NULL) {
+                                                   const std::string& path)
+    : service_(service), path_(path), value_(NULL) {
   DCHECK(service_->CalledOnValidThread());
 }