[cc] Adds source_id and sequence_number to BeginFrameArgs.

cc/output/begin_frame_args.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 #define CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 
+#include <stdint.h>
 #include <memory>
 
 #include "base/location.h"
 #include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "cc/base/cc_export.h"
 
 namespace base {
 namespace trace_event {
@@ skipping 24 matching lines @@
   enum BeginFrameArgsType {
     INVALID,
     NORMAL,
     MISSED,
     // Not a real type, but used by the IPC system. Should always remain the
     // *last* value in this enum.
     BEGIN_FRAME_ARGS_TYPE_MAX,
   };
   static const char* TypeToString(BeginFrameArgsType type);
 
+  static constexpr uint64_t kInvalidFrameNumber = 0;
+  static constexpr uint64_t kStartingFrameNumber = 1;
+
   // Creates an invalid set of values.
   BeginFrameArgs();
 
 #ifdef NDEBUG
   typedef const void* CreationLocation;
 #else
   typedef const tracked_objects::Location& CreationLocation;
   tracked_objects::Location created_from;
 #endif
 
   // You should be able to find all instances where a BeginFrame has been
   // created by searching for "BeginFrameArgs::Create".
   // The location argument should **always** be BEGINFRAME_FROM_HERE macro.
   static BeginFrameArgs Create(CreationLocation location,
+                               uint32_t source_id,
+                               uint64_t sequence_number,
                                base::TimeTicks frame_time,
                                base::TimeTicks deadline,
                                base::TimeDelta interval,
                                BeginFrameArgsType type);
 
   // This is the default delta that will be used to adjust the deadline when
   // proper draw-time estimations are not yet available.
   static base::TimeDelta DefaultEstimatedParentDrawTime();
 
   // This is the default interval to use to avoid sprinkling the code with
   // magic numbers.
   static base::TimeDelta DefaultInterval();
 
   bool IsValid() const { return interval >= base::TimeDelta(); }
 
   std::unique_ptr<base::trace_event::ConvertableToTraceFormat> AsValue() const;
   void AsValueInto(base::trace_event::TracedValue* dict) const;
 
   base::TimeTicks frame_time;
   base::TimeTicks deadline;
   base::TimeDelta interval;
+
+  // |source_id| and |sequence_number| identify a BeginFrame within a single
+  // process and are set by the original BeginFrameSource that created the
+  // BeginFrameArgs. When |source_id| of consecutive BeginFrameArgs changes,
+  // observers should expect the continuity of |sequence_number| to break.
+  uint64_t sequence_number;
+  uint32_t source_id;

[nasko, 2016/12/21 17:31:45]

nit: order the same way you have them in the constructor and the Create method.

[Eric Seckler, 2016/12/22 09:20:27]

The ordering here is taking packing of the struct into account, see earlier
comments on the patch by Sami. I'll leave the ordering in the constructor /
Create method as-is, since it makes more sense semantically, though. Updating
the order of fields/methods in ipc-related files to match field order.

Added a short comment here as well.

+
   BeginFrameArgsType type;
   bool on_critical_path;
 
  private:
-  BeginFrameArgs(base::TimeTicks frame_time,
+  BeginFrameArgs(uint32_t source_id,
+                 uint64_t sequence_number,
+                 base::TimeTicks frame_time,
                  base::TimeTicks deadline,
                  base::TimeDelta interval,
                  BeginFrameArgsType type);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_BEGIN_FRAME_ARGS_H_