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..d1ca08e899b117b76614c4e00bb04b46aaadab7e 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -48,6 +48,7 @@
 #include "cc/layers/scrollbar_layer_impl_base.h"
 #include "cc/layers/surface_layer_impl.h"
 #include "cc/layers/viewport.h"
+#include "cc/output/begin_frame_args.h"
 #include "cc/output/compositor_frame.h"
 #include "cc/output/compositor_frame_metadata.h"
 #include "cc/output/compositor_frame_sink.h"
@@ -179,7 +180,10 @@ 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(BeginFrameArgs::kInvalidFrameNumber),
+      latest_confirmed_frame(BeginFrameArgs::kInvalidFrameNumber) {}
 
 LayerTreeHostImpl::FrameData::~FrameData() {}
 
@@ -1658,6 +1662,10 @@ bool LayerTreeHostImpl::DrawLayers(FrameData* frame) {
     }
   }
 
+  DCHECK_LT(BeginFrameArgs::kInvalidFrameNumber, frame->begin_frame_number);
+  metadata.begin_frame_source_id = frame->begin_frame_source_id;
+  metadata.begin_frame_number = frame->begin_frame_number;
+  metadata.latest_confirmed_frame = frame->latest_confirmed_frame;
 
   CompositorFrame compositor_frame;
   compositor_frame.metadata = std::move(metadata);