cc: Fix DisplayScheduler:pending_swaps tracing.

cc/surfaces/display_scheduler.h

 // 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.
 
 #ifndef CC_SURFACES_DISPLAY_SCHEDULER_H_
 #define CC_SURFACES_DISPLAY_SCHEDULER_H_
 
 #include <memory>
 
 #include "base/cancelable_callback.h"
@@ skipping 66 matching lines @@
 
   bool visible_;
   bool output_surface_lost_;
   bool root_surface_resources_locked_;
 
   bool inside_begin_frame_deadline_interval_;
   bool needs_draw_;
   bool expecting_root_surface_damage_because_of_resize_;
   bool all_active_child_surfaces_ready_to_draw_;
 
+  int next_swap_id_;
   int pending_swaps_;
   int max_pending_swaps_;
 
   bool observing_begin_frame_source_;
 
   SurfaceId root_surface_id_;
   bool root_surface_damaged_;
   bool expect_damage_from_root_surface_;
 
   std::set<SurfaceId> child_surface_ids_damaged_;
   std::set<SurfaceId> child_surface_ids_damaged_prev_;
   std::vector<SurfaceId> child_surface_ids_to_expect_damage_from_;
 
   base::WeakPtrFactory<DisplayScheduler> weak_ptr_factory_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(DisplayScheduler);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_DISPLAY_SCHEDULER_H_