Revert of Remove default begin frame sources from the renderer

cc/trees/proxy_impl.cc

Index: cc/trees/proxy_impl.cc
diff --git a/cc/trees/proxy_impl.cc b/cc/trees/proxy_impl.cc
index 80adffbe0697a61a90e1932ea0fb8b0515dd947e..3a6f2db319847a10c0e1d43e28c366f80a0bb220 100644
--- a/cc/trees/proxy_impl.cc
+++ b/cc/trees/proxy_impl.cc
@@ -77,20 +77,24 @@
           CompositorTimingHistory::RENDERER_UMA,
           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(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_ = Scheduler::Create(this, scheduler_settings, layer_tree_host_id_,
-                                 task_runner_provider_->ImplThreadTaskRunner(),
-                                 external_begin_frame_source_.get(),
-                                 std::move(compositor_timing_history));
+  BeginFrameSource* 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_->ImplThreadTaskRunner())));
+    frame_source = unthrottled_begin_frame_source_.get();
+  }
+  if (!frame_source) {
+    synthetic_begin_frame_source_.reset(
+        new DelayBasedBeginFrameSource(base::MakeUnique<DelayBasedTimeSource>(
+            task_runner_provider_->ImplThreadTaskRunner())));
+    frame_source = synthetic_begin_frame_source_.get();
+  }
+  scheduler_ =
+      Scheduler::Create(this, scheduler_settings, layer_tree_host_id_,
+                        task_runner_provider_->ImplThreadTaskRunner(),
+                        frame_source, std::move(compositor_timing_history));
 
   DCHECK_EQ(scheduler_->visible(), layer_tree_host_impl_->visible());
 }
@@ -112,6 +116,8 @@
 
   scheduler_ = nullptr;
   external_begin_frame_source_ = nullptr;
+  unthrottled_begin_frame_source_ = nullptr;
+  synthetic_begin_frame_source_ = nullptr;
   layer_tree_host_impl_ = nullptr;
   // We need to explicitly shutdown the notifier to destroy any weakptrs it is
   // holding while still on the compositor thread. This also ensures any
@@ -269,6 +275,13 @@
   DCHECK(IsImplThread());
   channel_impl_->DidLoseOutputSurface();
   scheduler_->DidLoseOutputSurface();
+}
+
+void ProxyImpl::CommitVSyncParameters(base::TimeTicks timebase,
+                                      base::TimeDelta interval) {
+  DCHECK(IsImplThread());
+  if (synthetic_begin_frame_source_)
+    synthetic_begin_frame_source_->OnUpdateVSyncParameters(timebase, interval);
 }
 
 void ProxyImpl::SetBeginFrameSource(BeginFrameSource* source) {