Replace TaskType::kUnthrottled with kInternalDefault in RenderWidget

The task runner is used only for RenderWidget::Close and RenderWidget::
DoDeferredClose, and I think unthorttled task runner is not necessary
here.

Note that the original task runner before kUnthrottled was
ThreadTaskRunnerHandle::Get().

We plan to eliminate kUnthrottled, kUnspecedTimer and kUnspecedLoading
and replace them with appropriate kInternal* task types.

Task type guideline: https://bit.ly/2vMAsQ4

Change-Id: Iac3487edef157ff1261cfdde0b318f49cf469176
Reviewed-on: https://chromium-review.googlesource.com/1055140Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557815}

content/renderer/render_widget.cc

@@ -505,7 +505,7 @@ RenderWidget* RenderWidget::CreateForFrame(
     return view->GetWidget();
   }
   scoped_refptr<base::SingleThreadTaskRunner> task_runner =
-      frame->GetTaskRunner(blink::TaskType::kUnthrottled);
+      frame->GetTaskRunner(blink::TaskType::kInternalDefault);
   scoped_refptr<RenderWidget> widget(
       g_create_render_widget
           ? g_create_render_widget(widget_routing_id, compositor_deps,