Bug 1600906 - Added const in UpdateRefcount. r=dom-workers-and-storage-reviewers,ytausky

Depends on D56008

Differential Revision: https://phabricator.services.mozilla.com/D56009

--HG--
extra : moz-landing-system : lando

dom/indexedDB/ActorsParent.cpp

@@ -4915,7 +4915,7 @@ class DatabaseConnection::UpdateRefcountFunction::DatabaseUpdateFunction final {
   CachedStatement mSelectStatement;
   CachedStatement mInsertStatement;
 
-  UpdateRefcountFunction* mFunction;
+  UpdateRefcountFunction* const mFunction;
 
   nsresult mErrorCode;