cc: Fix DisplayScheduler:pending_swaps tracing.

cc/surfaces/display_scheduler.cc

 // Copyright 2015 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.
 
 #include "cc/surfaces/display_scheduler.h"
 
 #include <vector>
 
 #include "base/auto_reset.h"
 #include "base/stl_util.h"
 #include "base/trace_event/trace_event.h"
 #include "cc/output/output_surface.h"
 
 namespace cc {
 
 DisplayScheduler::DisplayScheduler(base::SingleThreadTaskRunner* task_runner,
                                    int max_pending_swaps)
     : client_(nullptr),
       begin_frame_source_(nullptr),
       task_runner_(task_runner),
       inside_surface_damaged_(false),
       visible_(false),
       output_surface_lost_(false),
       root_surface_resources_locked_(true),
       inside_begin_frame_deadline_interval_(false),
       needs_draw_(false),
       expecting_root_surface_damage_because_of_resize_(false),
       all_active_child_surfaces_ready_to_draw_(false),
+      next_swap_id_(1),
       pending_swaps_(0),
       max_pending_swaps_(max_pending_swaps),
       observing_begin_frame_source_(false),
       root_surface_damaged_(false),
       expect_damage_from_root_surface_(false),
       weak_ptr_factory_(this) {
   begin_frame_deadline_closure_ = base::Bind(
       &DisplayScheduler::OnBeginFrameDeadline, weak_ptr_factory_.GetWeakPtr());
 }
 
@@ skipping 326 matching lines @@
 void DisplayScheduler::DidFinishFrame(bool did_draw) {
   // TODO(eseckler): Determine and set correct |ack.latest_confirmed_frame|.
   BeginFrameAck ack(current_begin_frame_args_.source_id,
                     current_begin_frame_args_.sequence_number,
                     current_begin_frame_args_.sequence_number, 0, did_draw);
   begin_frame_source_->DidFinishFrame(this, ack);
 }
 
 void DisplayScheduler::DidSwapBuffers() {
   pending_swaps_++;
-  TRACE_EVENT_ASYNC_BEGIN1("cc", "DisplayScheduler:pending_swaps", this,
-                           "pending_frames", pending_swaps_);
+  uint32_t swap_id = next_swap_id_++;
+  TRACE_EVENT_ASYNC_BEGIN0("cc", "DisplayScheduler:pending_swaps", swap_id);
 }
 
 void DisplayScheduler::DidReceiveSwapBuffersAck() {
+  uint32_t swap_id = next_swap_id_ - pending_swaps_;
   pending_swaps_--;
-  TRACE_EVENT_ASYNC_END1("cc", "DisplayScheduler:pending_swaps", this,
-                         "pending_frames", pending_swaps_);
+  TRACE_EVENT_ASYNC_END0("cc", "DisplayScheduler:pending_swaps", swap_id);
   ScheduleBeginFrameDeadline();
 }
 
 }  // namespace cc