Get rid of the ability to unregister preferences.

base/prefs/pref_service.cc

Index: base/prefs/pref_service.cc
diff --git a/base/prefs/pref_service.cc b/base/prefs/pref_service.cc
index 04daed48b2096695af3b14558e54d0bc95cbde18..457778f37e4aafb1331484b01ad45e10c146b072 100644
--- a/base/prefs/pref_service.cc
+++ b/base/prefs/pref_service.cc
@@ -58,9 +58,6 @@ PrefService::PrefService(
   pref_registry_->SetRegistrationCallback(
       base::Bind(&PrefService::AddRegisteredPreference,
                  base::Unretained(this)));
-  pref_registry_->SetUnregistrationCallback(
-      base::Bind(&PrefService::RemoveRegisteredPreference,
-                 base::Unretained(this)));
   AddInitialPreferences();
 
   InitFromStorage(async);
@@ -69,10 +66,8 @@ PrefService::PrefService(
 PrefService::~PrefService() {
   DCHECK(CalledOnValidThread());
 
-  // Remove our callbacks, setting NULL ones.
+  // Remove our callback, setting a NULL one.
   pref_registry_->SetRegistrationCallback(PrefRegistry::RegistrationCallback());
-  pref_registry_->SetUnregistrationCallback(
-      PrefRegistry::UnregistrationCallback());
 
   // Reset pointers so accesses after destruction reliably crash.
   pref_value_store_.reset();
@@ -356,14 +351,6 @@ void PrefService::AddRegisteredPreference(const char* path,
     user_pref_store_->MarkNeedsEmptyValue(path);
 }
 
-// TODO(joi): We can get rid of this once the ability to unregister
-// prefs has been removed.
-void PrefService::RemoveRegisteredPreference(const char* path) {
-  DCHECK(CalledOnValidThread());
-
-  prefs_map_.erase(path);
-}
-
 void PrefService::ClearPref(const char* path) {
   DCHECK(CalledOnValidThread());