cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/output/begin_frame_args.h

Index: cc/output/begin_frame_args.h
diff --git a/cc/output/begin_frame_args.h b/cc/output/begin_frame_args.h
index 78ddce99ddfcde3c4508f1d706d4b1eb205ff89f..9352b1c890b62dca5d2e3a5dca5a9202718a1f3d 100644
--- a/cc/output/begin_frame_args.h
+++ b/cc/output/begin_frame_args.h
@@ -5,6 +5,8 @@
 #ifndef CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 #define CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 
+#include <stdint.h>
+
 #include <memory>
 
 #include "base/location.h"
@@ -63,6 +65,8 @@ struct CC_EXPORT BeginFrameArgs {
   // created by searching for "BeginFrameArgs::Create".
   // The location argument should **always** be BEGINFRAME_FROM_HERE macro.
   static BeginFrameArgs Create(CreationLocation location,
+                               uint64_t source_id,
+                               uint64_t sequence_number,
                                base::TimeTicks frame_time,
                                base::TimeTicks deadline,
                                base::TimeDelta interval,
@@ -81,6 +85,8 @@ struct CC_EXPORT BeginFrameArgs {
   std::unique_ptr<base::trace_event::ConvertableToTraceFormat> AsValue() const;
   void AsValueInto(base::trace_event::TracedValue* dict) const;
 
+  uint64_t source_id;
+  uint64_t sequence_number;
   base::TimeTicks frame_time;
   base::TimeTicks deadline;
   base::TimeDelta interval;
@@ -88,12 +94,50 @@ struct CC_EXPORT BeginFrameArgs {
   bool on_critical_path;
 
  private:
-  BeginFrameArgs(base::TimeTicks frame_time,
+  BeginFrameArgs(uint64_t source_id,
+                 uint64_t sequence_number,
+                 base::TimeTicks frame_time,
                  base::TimeTicks deadline,
                  base::TimeDelta interval,
                  BeginFrameArgsType type);
 };
 
+// Sent by a BeginFrameObserver as acknowledgment of completing a BeginFrame.
+struct CC_EXPORT BeginFrameAck {
+  BeginFrameAck();
+  BeginFrameAck(uint64_t source_id,
+                uint64_t sequence_number,
+                uint64_t oldest_incorporated_frame,
+                uint32_t remaining_frames,
+                bool has_updates);
+
+  // Source identifier of the BeginFrame that is acknowledged.
+  uint64_t source_id;
+
+  // Sequence number of the BeginFrame that is acknowledged.
+  uint64_t sequence_number;
+
+  // Sequence number of the oldest frame that contributed contents to the last
+  // update the consumer sent. It describes the "staleness" of the last update.
+  // Note that the last update may remain "fresh" even if the consumer did not
+  // produce an update for this frame (|has_updates == false|): If the consumer
+  // does not have any pending updates for a frame, the frame will be considered
+  // to have contributed its contents to the last update.
+  //
+  // The compositor Scheduler indicates the latest frame that both an impl and
+  // main frame contributed to (or didn't have any updates). The
+  // DisplayScheduler indicates the minimum oldest_incorporated_frame that all
+  // its BeginFrameObservers acknowledged.
+  uint64_t oldest_incorporated_frame;
+
+  // Number of BeginFrames queued at the observer at time of acknowledgment.
+  uint32_t remaining_frames;
+
+  // |true| if the consumer has produced updates (e.g. sent a CompositorFrame or
+  // damaged a surface) as part of responding to the BeginFrame.
+  bool has_updates;
+};
+
 }  // namespace cc
 
 #endif  // CC_OUTPUT_BEGIN_FRAME_ARGS_H_