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

components/sync/engine_impl/commit_contributor.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SYNC_ENGINE_IMPL_COMMIT_CONTRIBUTOR_H_
 #define COMPONENTS_SYNC_ENGINE_IMPL_COMMIT_CONTRIBUTOR_H_
 
 #include <cstddef>
 #include <memory>
 
 
 namespace syncer {
 
 class CommitContribution;
 
 namespace syncable {
 class Directory;
 }
 
 // This class represents a source of items to commit to the sync server.
 //
 // When asked, it can return CommitContribution objects that contain a set of
 // items to be committed from this source.
 class CommitContributor {
  public:
   CommitContributor();
   virtual ~CommitContributor() = 0;
 
   // Gathers up to |max_entries| unsynced items from this contributor into a
-  // CommitContribution.  Returns NULL when the contributor has nothing to
+  // CommitContribution.  Returns null when the contributor has nothing to
   // contribute.
   virtual std::unique_ptr<CommitContribution> GetContribution(
       size_t max_entries) = 0;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_IMPL_COMMIT_CONTRIBUTOR_H_