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

components/sync/engine_impl/directory_commit_contribution.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_DIRECTORY_COMMIT_CONTRIBUTION_H_
 #define COMPONENTS_SYNC_ENGINE_IMPL_DIRECTORY_COMMIT_CONTRIBUTION_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 24 matching lines @@
 // including processing the commit response message and setting and unsetting
 // the SYNCING bits.
 class DirectoryCommitContribution : public CommitContribution {
  public:
   // This destructor will DCHECK if UnsetSyncingBits() has not been called yet.
   ~DirectoryCommitContribution() override;
 
   // Build a CommitContribution from the IS_UNSYNCED items in |dir| with the
   // given |type|.  The contribution will include at most |max_items| entries.
   //
-  // This function may return NULL if this type has no items ready for and
+  // This function may return null if this type has no items ready for and
   // requiring commit.  This function may make model neutral changes to the
   // directory.
   static std::unique_ptr<DirectoryCommitContribution> Build(
       syncable::Directory* dir,
       ModelType type,
       size_t max_items,
       DirectoryTypeDebugInfoEmitter* debug_info_emitter);
 
   // Serialize this contribution's entries to the given commit request |msg|.
   //
@@ skipping 47 matching lines @@
 
   // A pointer to the commit counters of our parent CommitContributor.
   DirectoryTypeDebugInfoEmitter* debug_info_emitter_;
 
   DISALLOW_COPY_AND_ASSIGN(DirectoryCommitContribution);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_IMPL_DIRECTORY_COMMIT_CONTRIBUTION_H_