[heap] Fix page promotions tests

src/heap/page-parallel-job.h

Index: src/heap/page-parallel-job.h
diff --git a/src/heap/page-parallel-job.h b/src/heap/page-parallel-job.h
index ad1d9b3e301fd224ef6b10fc868408494dfcc572..fffbbd2e4004feb27f42d1329e1dfe1a348b20cd 100644
--- a/src/heap/page-parallel-job.h
+++ b/src/heap/page-parallel-job.h
@@ -63,10 +63,10 @@ class PageParallelJob {
     ++num_items_;
   }
 
-  int NumberOfPages() { return num_items_; }
+  int NumberOfPages() const { return num_items_; }
 
   // Returns the number of tasks that were spawned when running the job.
-  int NumberOfTasks() { return num_tasks_; }
+  int NumberOfTasks() const { return num_tasks_; }
 
   // Runs the given number of tasks in parallel and processes the previously
   // added pages. This function blocks until all tasks finish.