cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index e27073c6351fc2341e27cb411ae7621e9c8730d3..6b97fa41c39059f64ad16da9580f5963fbbbb35d 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -179,7 +179,10 @@ DEFINE_SCOPED_UMA_HISTOGRAM_TIMER(PendingTreeDurationHistogramTimer,
 LayerTreeHostImpl::FrameData::FrameData()
     : render_surface_layer_list(nullptr),
       has_no_damage(false),
-      may_contain_video(false) {}
+      may_contain_video(false),
+      begin_frame_source_id(0),
+      begin_frame_number(0),
+      oldest_incorporated_frame(0) {}
 
 LayerTreeHostImpl::FrameData::~FrameData() {}
 
@@ -1658,6 +1661,9 @@ bool LayerTreeHostImpl::DrawLayers(FrameData* frame) {
     }
   }
 
+  metadata.begin_frame_source_id = frame->begin_frame_source_id;
+  metadata.begin_frame_number = frame->begin_frame_number;
+  metadata.oldest_incorporated_frame = frame->oldest_incorporated_frame;
 
   CompositorFrame compositor_frame;
   compositor_frame.metadata = std::move(metadata);