PostTask in ServiceWorkerContextWrapper::StopAllServiceWorkersOnIO()

This is a following CL for https://chromium-review.googlesource.com/c/chromium/src/+/635105/23
|callback| should be run from |task_runner_for_callback|.

Bug: 736697
Change-Id: I51212a96f00801fd019670ca6b3c57d755b3d5c6
Reviewed-on: https://chromium-review.googlesource.com/672265
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Yuki Yamada <yukiy@google.com>
Cr-Commit-Position: refs/heads/master@{#503329}

content/browser/service_worker/service_worker_context_wrapper.cc

@@ -1049,8 +1049,11 @@ void ServiceWorkerContextWrapper::StopAllServiceWorkersOnIO(
     return;
   }
   std::vector<ServiceWorkerVersionInfo> live_versions = GetAllLiveVersionInfo();
-  base::RepeatingClosure barrier =
-      base::BarrierClosure(live_versions.size(), std::move(callback));
+  base::RepeatingClosure barrier = base::BarrierClosure(
+      live_versions.size(),
+      base::BindOnce(
+          base::IgnoreResult(&base::SingleThreadTaskRunner::PostTask),
+          std::move(task_runner_for_callback), FROM_HERE, std::move(callback)));
   for (const ServiceWorkerVersionInfo& info : live_versions) {
     ServiceWorkerVersion* version = GetLiveVersion(info.version_id);
     DCHECK(version);