scheduler: Return a base::SingleThreadTaskRunner for the default task runner.

content/test/fake_renderer_scheduler.cc

Index: content/test/fake_renderer_scheduler.cc
diff --git a/content/test/fake_renderer_scheduler.cc b/content/test/fake_renderer_scheduler.cc
index a12f25729a8dd88e77d383f919e70633b6e0330b..e1e490337918431fc4a19682450550bc90abe8c3 100644
--- a/content/test/fake_renderer_scheduler.cc
+++ b/content/test/fake_renderer_scheduler.cc
@@ -18,7 +18,8 @@ scoped_ptr<blink::WebThread> FakeRendererScheduler::CreateMainThread() {
   return nullptr;
 }
 
-scoped_refptr<scheduler::TaskQueue> FakeRendererScheduler::DefaultTaskRunner() {
+scoped_refptr<base::SingleThreadTaskRunner>
+FakeRendererScheduler::DefaultTaskRunner() {
   return nullptr;
 }