Bug 1452898 - Make count of helper thread zones atomic as this can be read without locking by helper threads r=jandem

js/src/vm/Runtime.h

@@ -479,7 +479,7 @@ struct JSRuntime : public js::MallocProvider<JSRuntime>
      * Number of zones which may be operated on by non-cooperating helper
      * threads.
      */
-    js::UnprotectedData<size_t> numActiveHelperThreadZones;
+    mozilla::Atomic<size_t> numActiveHelperThreadZones;
 
     friend class js::AutoLockForExclusiveAccess;
     friend class js::AutoLockScriptData;