Use the given task runner for ThrottlingURLLoader instead of per-thread task runner

[Context]
This is a part of an effort to achieve MBI. We want to avoid per-thread task runner as much as possible.

[Previous code]
ThrottlingURLLoader::url_loader_ was using per-thread default task runner because task_runner was not specified for BindNewPipeAndPassReceiver.

[What this CL does]
Use the given task runner for ThrottlingURLLoader::url_loader_ instead of per-thread task runner

Bug: 1105403
Change-Id: I7da84124ac5f7adfea5327f3b976f8e45cb5d547
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463262Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816033}

third_party/blink/common/loader/throttling_url_loader.cc

@@ -523,8 +523,9 @@ void ThrottlingURLLoader::StartNow() {
 
   DCHECK(start_info_->url_loader_factory);
   start_info_->url_loader_factory->CreateLoaderAndStart(
-      url_loader_.BindNewPipeAndPassReceiver(), start_info_->routing_id,
-      start_info_->request_id, start_info_->options, start_info_->url_request,
+      url_loader_.BindNewPipeAndPassReceiver(start_info_->task_runner),
+      start_info_->routing_id, start_info_->request_id, start_info_->options,
+      start_info_->url_request,
       client_receiver_.BindNewPipeAndPassRemote(start_info_->task_runner),
       net::MutableNetworkTrafficAnnotationTag(traffic_annotation_));