Let IndexedDBContextImpl create its own task runner

content/browser/indexed_db/indexed_db_dispatcher_host.h

Index: content/browser/indexed_db/indexed_db_dispatcher_host.h
diff --git a/content/browser/indexed_db/indexed_db_dispatcher_host.h b/content/browser/indexed_db/indexed_db_dispatcher_host.h
index c56d8c2bb8736bf092c71c70ffe879a3664b9e9b..1eaa199865634fed7434471feed82c1e464a829a 100644
--- a/content/browser/indexed_db/indexed_db_dispatcher_host.h
+++ b/content/browser/indexed_db/indexed_db_dispatcher_host.h
@@ -109,9 +109,10 @@ class CONTENT_EXPORT IndexedDBDispatcherHost
 
   void InvalidateWeakPtrsAndClearBindings();
 
+  base::SequencedTaskRunner* IDBTaskRunner() const;
+
   scoped_refptr<IndexedDBContextImpl> indexed_db_context_;
   scoped_refptr<ChromeBlobStorageContext> blob_storage_context_;
-  scoped_refptr<base::SequencedTaskRunner> idb_runner_;
 
   // Maps blob uuid to a pair (handle, ref count). Entry is added and/or count
   // is incremented in HoldBlobData(), and count is decremented and/or entry