compositor worker: Use a WebThread for the compositor thread.

components/scheduler/child/worker_scheduler_impl.cc

Index: components/scheduler/child/worker_scheduler_impl.cc
diff --git a/components/scheduler/child/worker_scheduler_impl.cc b/components/scheduler/child/worker_scheduler_impl.cc
index e74328d10fc650bc622974b4381bf43a34db11f5..e02bb7ea47b53d4fb2824835978681ff9e1a18da 100644
--- a/components/scheduler/child/worker_scheduler_impl.cc
+++ b/components/scheduler/child/worker_scheduler_impl.cc
@@ -39,7 +39,8 @@ void WorkerSchedulerImpl::Init() {
   idle_helper_.EnableLongIdlePeriod();
 }
 
-scoped_refptr<TaskQueue> WorkerSchedulerImpl::DefaultTaskRunner() {
+scoped_refptr<base::SingleThreadTaskRunner>
+WorkerSchedulerImpl::DefaultTaskRunner() {
   DCHECK(initialized_);
   return helper_.DefaultTaskRunner();
 }