Remove default begin frame sources from the renderer

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 cf9206c939011ef6b4965eb93e2b2745ee3abb70..91ffe05b533273f1762d7181a88da0cd930b5371 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -74,28 +74,22 @@ void SingleThreadProxy::Start(
             CompositorTimingHistory::BROWSER_UMA,
             layer_tree_host_->rendering_stats_instrumentation()));
 
-    BeginFrameSource* frame_source = nullptr;
-    if (!layer_tree_host_->settings().use_output_surface_begin_frame_source) {
-      frame_source = external_begin_frame_source_.get();
-      if (!scheduler_settings.throttle_frame_production) {
-        // Unthrottled source takes precedence over external sources.
-        unthrottled_begin_frame_source_.reset(new BackToBackBeginFrameSource(
-            base::MakeUnique<DelayBasedTimeSource>(
-                task_runner_provider_->MainThreadTaskRunner())));
-        frame_source = unthrottled_begin_frame_source_.get();
-      }
-      if (!frame_source) {
-        synthetic_begin_frame_source_.reset(new DelayBasedBeginFrameSource(
-            base::MakeUnique<DelayBasedTimeSource>(
-                task_runner_provider_->MainThreadTaskRunner())));
-        frame_source = synthetic_begin_frame_source_.get();
-      }
-    }
-
+    // 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(layer_tree_host_->settings().use_external_begin_frame_source ^
+           layer_tree_host_->settings().use_output_surface_begin_frame_source);
+    DCHECK(!layer_tree_host_->settings().use_external_begin_frame_source ||
+           external_begin_frame_source_);
+    DCHECK(
+        !layer_tree_host_->settings().use_output_surface_begin_frame_source ||
+        !external_begin_frame_source_);
     scheduler_on_impl_thread_ =
         Scheduler::Create(this, scheduler_settings, layer_tree_host_->id(),
                           task_runner_provider_->MainThreadTaskRunner(),
-                          frame_source, std::move(compositor_timing_history));
+                          external_begin_frame_source_.get(),
+                          std::move(compositor_timing_history));
   }
 
   layer_tree_host_impl_ = layer_tree_host_->CreateLayerTreeHostImpl(this);