Route resource dispatch tasks via the loading task queue

This patch should make the main thread more responsive when the user
is interacting with the device while the page is loading.

Sherrifs if you see an uptick in blink layout test flakes this patch
might be to blame.

I tried to reproduce the flakes I saw in the try runs locally.
With the exception of fast/loader/subframe-removes-itself.html
I was not able to reproduce a flake, however judging by the flakiness
dashboard, that test is already slightly flaky.

BUG=391005

Review URL: https://codereview.chromium.org/882073002

Cr-Commit-Position: refs/heads/master@{#313739}

content/renderer/render_thread_impl.cc

@@ -991,8 +991,7 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
       base::Bind(base::IgnoreResult(&RenderThreadImpl::OnMessageReceived),
                  base::Unretained(this)));
 
-  // TODO(alexclarke): Add a dedicated loading task queue
-  SetResourceDispatchTaskQueue(renderer_scheduler_->DefaultTaskRunner());
+  SetResourceDispatchTaskQueue(renderer_scheduler_->LoadingTaskRunner());
 
   bool enable = !command_line.HasSwitch(switches::kDisableThreadedCompositing);
   if (enable) {