[cc] Fix CompositorFrameSinkSupport BeginFrameAck interface.

services/ui/ws/frame_generator.cc

Index: services/ui/ws/frame_generator.cc
diff --git a/services/ui/ws/frame_generator.cc b/services/ui/ws/frame_generator.cc
index 30b17f3da99bf188dcddf76cbfd9508f4e87a110..d081b84c33c96ee3f2fb0d536406e5ab9f44c160 100644
--- a/services/ui/ws/frame_generator.cc
+++ b/services/ui/ws/frame_generator.cc
@@ -93,15 +93,16 @@ void FrameGenerator::SetExternalTilePriorityConstraints(
     const gfx::Transform& transform) {}
 
 void FrameGenerator::OnBeginFrame(const cc::BeginFrameArgs& begin_frame_args) {
+  // TODO(eseckler): Acknowledge BeginFrame if we don't submit CompositorFrame.
   if (!root_window_->visible())
     return;
+  last_begin_frame_args_ = begin_frame_args;
 
   // TODO(fsamuel): We should add a trace for generating a top level frame.
   cc::CompositorFrame frame(GenerateCompositorFrame(root_window_->bounds()));
 
   compositor_frame_sink_->SubmitCompositorFrame(std::move(frame));
   SetNeedsBeginFrame(false);
-  last_begin_frame_args_ = begin_frame_args;
 }
 
 const cc::BeginFrameArgs& FrameGenerator::LastUsedBeginFrameArgs() const {
@@ -142,6 +143,9 @@ cc::CompositorFrame FrameGenerator::GenerateCompositorFrame(
     frame.render_pass_list.push_back(std::move(invert_pass));
   }
   frame.metadata.device_scale_factor = device_scale_factor_;
+  frame.metadata.begin_frame_ack = cc::BeginFrameAck(
+      last_begin_frame_args_.source_id, last_begin_frame_args_.sequence_number,
+      last_begin_frame_args_.sequence_number, 0, true);
 
   if (window_manager_surface_info_.is_valid()) {
     frame.metadata.referenced_surfaces.push_back(