Use SequencedWorkerPoolOwner in MemoryDumpManagerTest.

base/test/sequenced_worker_pool_owner.h

Index: base/test/sequenced_worker_pool_owner.h
diff --git a/base/test/sequenced_worker_pool_owner.h b/base/test/sequenced_worker_pool_owner.h
index 05fc7505fb5068361ad6946efc442424bfe401d4..28a6cf070a2d462218c79862f77f9cd7e4613e3c 100644
--- a/base/test/sequenced_worker_pool_owner.h
+++ b/base/test/sequenced_worker_pool_owner.h
@@ -39,7 +39,7 @@ class SequencedWorkerPoolOwner : public SequencedWorkerPool::TestingObserver {
   ~SequencedWorkerPoolOwner() override;
 
   // Don't change the returned pool's testing observer.
-  const scoped_refptr<SequencedWorkerPool>& pool();
+  const scoped_refptr<SequencedWorkerPool>& pool() const;
 
   // The given callback will be called on WillWaitForShutdown().
   void SetWillWaitForShutdownCallback(const Closure& callback);