Remove uses of external begin frame sources

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index dc46c48e6436415e03eebe234b3b803abdc60608..41b47fa291cd965152576e8bee7a873dc10b1524 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -74,16 +74,9 @@ void SingleThreadProxy::Start(
             CompositorTimingHistory::BROWSER_UMA,
             layer_tree_host_->rendering_stats_instrumentation()));
 
-    // BFS must either be external or come from the output surface.  If
-    // external, it must be provided.  If from the output surface, it must
-    // not be provided.
-    // TODO(enne): Make all BFS come from the output surface.
-    DCHECK(settings.use_external_begin_frame_source ^
-           settings.use_output_surface_begin_frame_source);
-    DCHECK(!settings.use_external_begin_frame_source ||
-           external_begin_frame_source_);
-    DCHECK(!settings.use_output_surface_begin_frame_source ||
-           !external_begin_frame_source_);
+    // TODO(enne): remove these settings.
+    DCHECK(!settings.use_external_begin_frame_source);
+    DCHECK(settings.use_output_surface_begin_frame_source);
     scheduler_on_impl_thread_ =
         Scheduler::Create(this, scheduler_settings, layer_tree_host_->GetId(),
                           task_runner_provider_->MainThreadTaskRunner(),