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

components/sync/syncable/model_neutral_mutable_entry.cc

Index: components/sync/syncable/model_neutral_mutable_entry.cc
diff --git a/components/sync/syncable/model_neutral_mutable_entry.cc b/components/sync/syncable/model_neutral_mutable_entry.cc
index 2a3e56cc05d8ec621382b66e38bde71793d738ce..900f243f31b071bc4b433b485e42a7446b1bd750 100644
--- a/components/sync/syncable/model_neutral_mutable_entry.cc
+++ b/components/sync/syncable/model_neutral_mutable_entry.cc
@@ -24,7 +24,7 @@ ModelNeutralMutableEntry::ModelNeutralMutableEntry(BaseWriteTransaction* trans,
                                                    const Id& id)
     : Entry(trans), base_write_transaction_(trans) {
   Entry same_id(trans, GET_BY_ID, id);
-  kernel_ = NULL;
+  kernel_ = nullptr;
   if (same_id.good()) {
     return;  // already have an item with this ID.
   }
@@ -53,7 +53,7 @@ ModelNeutralMutableEntry::ModelNeutralMutableEntry(BaseWriteTransaction* trans,
   if (type != NIGORI)
     DCHECK(IsTypeWithClientGeneratedRoot(type));
   Entry same_type_root(trans, GET_TYPE_ROOT, type);
-  kernel_ = NULL;
+  kernel_ = nullptr;
   if (same_type_root.good()) {
     return;  // already have a type root for the given type
   }