cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 2bbf7825465110fb2f64438ac05817790f2ef71a..cba0a6a5ae5b411a0d928d8e1a99c652fcdba0cc 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -25,7 +25,6 @@
 #include "cc/input/scrollbar_animation_controller.h"
 #include "cc/layers/layer_collections.h"
 #include "cc/layers/render_pass_sink.h"
-#include "cc/output/begin_frame_args.h"
 #include "cc/output/compositor_frame_sink_client.h"
 #include "cc/output/context_cache_controller.h"
 #include "cc/output/managed_memory_policy.h"
@@ -223,6 +222,9 @@ class CC_EXPORT LayerTreeHostImpl
     LayerImplList will_draw_layers;
     bool has_no_damage;
     bool may_contain_video;
+    uint64_t begin_frame_source_id;
+    uint64_t begin_frame_number;
+    uint64_t latest_confirmed_frame;
 
     // RenderPassSink implementation.
     void AppendRenderPass(std::unique_ptr<RenderPass> render_pass) override;