Introduce ChildScheduler as common base interface for WorkerScheduler and RendererScheduler.

content/child/scheduler/null_worker_scheduler.cc

Index: content/child/scheduler/null_worker_scheduler.cc
diff --git a/content/child/scheduler/null_worker_scheduler.cc b/content/child/scheduler/null_worker_scheduler.cc
index 5949bbd7f65bd0d6c6d4dbce315d6f4d110ed471..3127c3f13370af5a36d02f1ff8a0da23e350d8e0 100644
--- a/content/child/scheduler/null_worker_scheduler.cc
+++ b/content/child/scheduler/null_worker_scheduler.cc
@@ -42,6 +42,10 @@ bool NullWorkerScheduler::CanExceedIdleDeadlineIfRequired() const {
   return false;
 }
 
+bool NullWorkerScheduler::ShouldYieldForHighPriorityWork() {
+  return false;
+}
+
 void NullWorkerScheduler::Init() {
 }