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

components/sync/syncable/base_transaction.h

Index: components/sync/syncable/base_transaction.h
diff --git a/components/sync/syncable/base_transaction.h b/components/sync/syncable/base_transaction.h
index 2aafe45d2e9df0dd8de9b540ad1a3d06c6b85afd..a85787cb703636bac8f044d242fde52173f61659 100644
--- a/components/sync/syncable/base_transaction.h
+++ b/components/sync/syncable/base_transaction.h
@@ -36,7 +36,7 @@ class BaseTransaction {
 
   syncable::Directory* GetDirectory() const {
     if (!user_share_) {
-      return NULL;
+      return nullptr;
     } else {
       return user_share_->directory.get();
     }
@@ -48,7 +48,7 @@ class BaseTransaction {
   explicit BaseTransaction(UserShare* share);
   virtual ~BaseTransaction();
 
-  BaseTransaction() : user_share_(NULL) {}
+  BaseTransaction() : user_share_(nullptr) {}
 
  private:
   UserShare* user_share_;