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

components/sync/syncable/write_transaction.cc

Index: components/sync/syncable/write_transaction.cc
diff --git a/components/sync/syncable/write_transaction.cc b/components/sync/syncable/write_transaction.cc
index fd01ce8496b97473332bee040234fd6b22dbc0a8..7eb0b5a8da3f2a284590b6c5149a7a2908e8d8d2 100644
--- a/components/sync/syncable/write_transaction.cc
+++ b/components/sync/syncable/write_transaction.cc
@@ -14,7 +14,7 @@ namespace syncer {
 // WriteTransaction member definitions
 WriteTransaction::WriteTransaction(const tracked_objects::Location& from_here,
                                    UserShare* share)
-    : BaseTransaction(share), transaction_(NULL) {
+    : BaseTransaction(share), transaction_(nullptr) {
   transaction_ = new syncable::WriteTransaction(from_here, syncable::SYNCAPI,
                                                 share->directory.get());
 }
@@ -22,7 +22,7 @@ WriteTransaction::WriteTransaction(const tracked_objects::Location& from_here,
 WriteTransaction::WriteTransaction(const tracked_objects::Location& from_here,
                                    UserShare* share,
                                    int64_t* new_model_version)
-    : BaseTransaction(share), transaction_(NULL) {
+    : BaseTransaction(share), transaction_(nullptr) {
   transaction_ = new syncable::WriteTransaction(
       from_here, share->directory.get(), new_model_version);
 }