cc: Introduce forward_begin_frames_to_children and use_external_begin_frame_source

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index 9ed3d7208efaeced0511908cb5b49664321e22b2..5c515b2512e210919d95dcd82593359861777cdf 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -29,7 +29,7 @@ BeginFrameSource* SchedulerFrameSourcesConstructor::ConstructPrimaryFrameSource(
     scheduler->primary_frame_source_internal_ =
         BackToBackBeginFrameSource::Create(scheduler->task_runner_.get());
     return scheduler->primary_frame_source_internal_.get();
-  } else if (scheduler->settings_.begin_frame_scheduling_enabled) {
+  } else if (scheduler->settings_.use_external_begin_frame_source) {
     TRACE_EVENT1("cc",
                  "Scheduler::Scheduler()",
                  "PrimaryFrameSource",