Change transaction in different scope to a readonly transaction.

Firefox doesn't allow running readwrite transactions in parallel, even with a
different scope. For the purpose of this particular test, it shouldn't matter
if it is a readwrite or readonly transaction.

Differential Revision: https://phabricator.services.mozilla.com/D55672

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1497007
gecko-commit: 38516e564ad923a54cea44209683245322dd3456
gecko-integration-branch: autoland
gecko-reviewers: dom-workers-and-storage-reviewers, asuth, janv

IndexedDB/idb-explicit-commit.any.js

@@ -196,9 +196,12 @@ promise_test(async testCase => {
   txn2.commit();
 
   // Exercise the IndexedDB transaction ordering by executing one with a
-  // different scope.
-  const txn3 = db.transaction(['not_books'], 'readwrite');
-  txn3.objectStore('not_books').put({'title': 'not_title'}, 'key');
+  // different scope. A readonly transaction is used here because
+  // implementations are not required to run non-overlapping readwrite
+  // transactions in parallel, and some implementations (ex: Firefox)
+  // will not.
+  const txn3 = db.transaction(['not_books'], 'readonly');
+  txn3.objectStore('not_books').getAllKeys();
   txn3.oncomplete = function() {
     releaseTxnFunction();
   }