Remove default begin frame sources from the renderer

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 1ea1f2a7e89fbab707959c29bba37c1cfde84478..6ba8d2aaec5b02daba1b2cd9b026ba3a9a39cf1c 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -243,6 +243,8 @@ void RenderWidgetCompositor::Initialize(float device_scale_factor) {
   if (settings.use_external_begin_frame_source) {
     params.external_begin_frame_source =
         delegate_->CreateExternalBeginFrameSource();
+  } else {
+    DCHECK(settings.use_output_surface_begin_frame_source);
   }
   params.animation_host = cc::AnimationHost::CreateMainInstance();
 
@@ -451,7 +453,14 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
   // TODO(danakj): Only do this on low end devices.
   settings.create_low_res_tiling = true;
 
-  settings.use_external_begin_frame_source = true;
+  // TODO(enne): fold external BFS into output surface BFS.
+  if (command_line->HasSwitch(switches::kUseRemoteCompositing)) {
+    settings.use_output_surface_begin_frame_source = true;
+    settings.use_external_begin_frame_source = false;
+  } else {
+    settings.use_output_surface_begin_frame_source = false;
+    settings.use_external_begin_frame_source = true;
+  }
 
 #else  // defined(OS_ANDROID)
 #if !defined(OS_MACOSX)
@@ -489,8 +498,6 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
     settings.create_low_res_tiling = true;
   if (cmd.HasSwitch(switches::kDisableLowResTiling))
     settings.create_low_res_tiling = false;
-  if (!cmd.HasSwitch(cc::switches::kDisableBeginFrameScheduling))
-    settings.use_external_begin_frame_source = true;
 
   if (cmd.HasSwitch(switches::kEnableRGBA4444Textures) &&
       !cmd.HasSwitch(switches::kDisableRGBA4444Textures)) {
@@ -512,10 +519,6 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
   settings.use_cached_picture_raster =
       !cmd.HasSwitch(cc::switches::kDisableCachedPictureRaster);
 
-  if (cmd.HasSwitch(switches::kUseRemoteCompositing) ||
-      cmd.HasSwitch(switches::kIsRunningInMash))
-    settings.use_external_begin_frame_source = false;
-
   return settings;
 }