[Sync] Replacing NULL with nullptr in code and null in comments for components/sync/

components/sync/base/sync_prefs.h

Index: components/sync/base/sync_prefs.h
diff --git a/components/sync/base/sync_prefs.h b/components/sync/base/sync_prefs.h
index a6572c2db79267b88a67c941c1ff7fa1b7d0ae0a..0343c7af50a681465fa59c4564c464df906fc517 100644
--- a/components/sync/base/sync_prefs.h
+++ b/components/sync/base/sync_prefs.h
@@ -57,7 +57,7 @@ class SyncPrefObserver {
 class SyncPrefs : NON_EXPORTED_BASE(public base::NonThreadSafe),
                   public base::SupportsWeakPtr<SyncPrefs> {
  public:
-  // |pref_service| may not be NULL.
+  // |pref_service| may not be null.
   // Does not take ownership of |pref_service|.
   explicit SyncPrefs(PrefService* pref_service);
 
@@ -198,7 +198,7 @@ class SyncPrefs : NON_EXPORTED_BASE(public base::NonThreadSafe),
 
   void OnSyncManagedPrefChanged();
 
-  // May be NULL.
+  // May be null.
   PrefService* const pref_service_;
 
   base::ObserverList<SyncPrefObserver> sync_pref_observers_;