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

components/sync/syncable/read_transaction.cc

Index: components/sync/syncable/read_transaction.cc
diff --git a/components/sync/syncable/read_transaction.cc b/components/sync/syncable/read_transaction.cc
index 896c432c71adc4f7b07ce460d42faf27b308b308..1055e45232ce63269be957f9bfda9b31583bedff 100644
--- a/components/sync/syncable/read_transaction.cc
+++ b/components/sync/syncable/read_transaction.cc
@@ -13,7 +13,7 @@ namespace syncer {
 // ReadTransaction member definitions
 ReadTransaction::ReadTransaction(const tracked_objects::Location& from_here,
                                  UserShare* share)
-    : BaseTransaction(share), transaction_(NULL), close_transaction_(true) {
+    : BaseTransaction(share), transaction_(nullptr), close_transaction_(true) {
   transaction_ =
       new syncable::ReadTransaction(from_here, share->directory.get());
 }