cc: Introduce forward_begin_frames_to_children and use_external_begin_frame_source

cc/scheduler/scheduler_settings.h

Index: cc/scheduler/scheduler_settings.h
diff --git a/cc/scheduler/scheduler_settings.h b/cc/scheduler/scheduler_settings.h
index 8607991914e80979b049e00693a4ad55325722c1..362a95e3aa2eb74ff588380d5ed21b40424e1845 100644
--- a/cc/scheduler/scheduler_settings.h
+++ b/cc/scheduler/scheduler_settings.h
@@ -24,7 +24,8 @@ class CC_EXPORT SchedulerSettings {
   explicit SchedulerSettings(const LayerTreeSettings& settings);
   ~SchedulerSettings();
 
-  bool begin_frame_scheduling_enabled;
+  bool begin_frame_receiver;
+  bool begin_frame_publisher;
   bool main_frame_before_activation_enabled;
   bool impl_side_painting;
   bool timeout_and_draw_when_animation_checkerboards;