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

components/sync/engine_impl/directory_commit_contribution.h

Index: components/sync/engine_impl/directory_commit_contribution.h
diff --git a/components/sync/engine_impl/directory_commit_contribution.h b/components/sync/engine_impl/directory_commit_contribution.h
index 9d9a177e680be7cc870272a9b653d572d6611500..c8eb3098f83600a992318233a1a7d38c506455a4 100644
--- a/components/sync/engine_impl/directory_commit_contribution.h
+++ b/components/sync/engine_impl/directory_commit_contribution.h
@@ -42,7 +42,7 @@ class DirectoryCommitContribution : public CommitContribution {
   // 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(