Hoist begin frame sources out of scheduler

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 3f5a01eee061add58aca0e8d3f89ef3df147bc05..395092fe91bc0dc8865a8f5ece969bde83da8207 100644
--- a/cc/trees/proxy_impl.cc
+++ b/cc/trees/proxy_impl.cc
@@ -60,7 +60,7 @@ ProxyImpl::ProxyImpl(ChannelImpl* channel_impl,
           base::Bind(&ProxyImpl::RenewTreePriority, base::Unretained(this)),
           base::TimeDelta::FromSecondsD(
               kSmoothnessTakesPriorityExpirationDelay)),
-      external_begin_frame_source_(std::move(external_begin_frame_source)),
+      begin_frame_source_(std::move(external_begin_frame_source)),
       rendering_stats_instrumentation_(
           layer_tree_host->rendering_stats_instrumentation()),
       channel_impl_(channel_impl) {
@@ -79,10 +79,22 @@ ProxyImpl::ProxyImpl(ChannelImpl* channel_impl,
           CompositorTimingHistory::RENDERER_UMA,
           rendering_stats_instrumentation_));
 
-  scheduler_ = Scheduler::Create(this, scheduler_settings, layer_tree_host_id_,
-                                 task_runner_provider_->ImplThreadTaskRunner(),
-                                 external_begin_frame_source_.get(),
-                                 std::move(compositor_timing_history));
+  if (!scheduler_settings.throttle_frame_production) {
+    // Unthrottled source takes precedence over external sources.
+    begin_frame_source_ = BackToBackBeginFrameSource::Create(
+        task_runner_provider_->ImplThreadTaskRunner());
+  }
+  BeginFrameSource* frame_source = begin_frame_source_.get();
+  if (!frame_source) {
+    synthetic_frame_source_ = SyntheticBeginFrameSource::Create(
+        task_runner_provider_->ImplThreadTaskRunner(),
+        BeginFrameArgs::DefaultInterval());
+    frame_source = synthetic_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());
 }
@@ -98,7 +110,8 @@ ProxyImpl::~ProxyImpl() {
   DCHECK(IsMainThreadBlocked());
 
   scheduler_ = nullptr;
-  external_begin_frame_source_ = nullptr;
+  begin_frame_source_ = nullptr;
+  synthetic_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
@@ -276,7 +289,14 @@ void ProxyImpl::DidLoseOutputSurfaceOnImplThread() {
 void ProxyImpl::CommitVSyncParameters(base::TimeTicks timebase,
                                       base::TimeDelta interval) {
   DCHECK(IsImplThread());
-  scheduler_->CommitVSyncParameters(timebase, interval);
+  if (!synthetic_frame_source_)
+    return;
+
+  if (interval == base::TimeDelta()) {
+    // TODO(brianderson): We should not be receiving 0 intervals.
+    interval = BeginFrameArgs::DefaultInterval();
+  }
+  synthetic_frame_source_->OnUpdateVSyncParameters(timebase, interval);
 }
 
 void ProxyImpl::SetEstimatedParentDrawTime(base::TimeDelta draw_time) {