Remove the last usages of PrefObserver outside of Prefs.

chrome/browser/autofill/autocomplete_history_manager.cc

Index: chrome/browser/autofill/autocomplete_history_manager.cc
diff --git a/chrome/browser/autofill/autocomplete_history_manager.cc b/chrome/browser/autofill/autocomplete_history_manager.cc
index 438113b9163cc24106570325344f0b9fba80dc01..cddf0a9b9d7d1a0c76281d78571b366ad2347e80 100644
--- a/chrome/browser/autofill/autocomplete_history_manager.cc
+++ b/chrome/browser/autofill/autocomplete_history_manager.cc
@@ -116,8 +116,7 @@ AutocompleteHistoryManager::AutocompleteHistoryManager(
   // May be NULL in unit tests.
   autofill_data_ = AutofillWebDataService::FromBrowserContext(browser_context_);
   autofill_enabled_.Init(prefs::kAutofillEnabled,
-                         PrefServiceBase::FromBrowserContext(browser_context_),
-                         NULL);
+                         PrefServiceBase::FromBrowserContext(browser_context_));
 }
 
 AutocompleteHistoryManager::~AutocompleteHistoryManager() {
@@ -244,8 +243,7 @@ AutocompleteHistoryManager::AutocompleteHistoryManager(
       query_id_(0),
       external_delegate_(NULL) {
   autofill_enabled_.Init(prefs::kAutofillEnabled,
-                         PrefServiceBase::FromBrowserContext(browser_context_),
-                         NULL);
+                         PrefServiceBase::FromBrowserContext(browser_context_));
 }
 
 void AutocompleteHistoryManager::CancelPendingQuery() {