[cc] Acknowledge BeginFrames in SurfaceDependencyTracker.

cc/surfaces/surface_dependency_tracker.h

 // Copyright 2017 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_SURFACE_DEPENDENCY_TRACKER_H_
 #define CC_SURFACES_SURFACE_DEPENDENCY_TRACKER_H_
 
 #include "cc/scheduler/begin_frame_source.h"
 #include "cc/surfaces/pending_frame_observer.h"
 #include "cc/surfaces/surface.h"
@@ skipping 53 matching lines @@
   // SurfaceObserver implementation:
   void OnSurfaceCreated(const SurfaceInfo& surface_info) override;
   void OnSurfaceDamaged(const SurfaceId& surface_id, bool* changed) override;
 
  private:
   // Informs all Surfaces with pending frames blocked on the provided
   // |surface_id| that there is now an active frame available in Surface
   // corresponding to |surface_id|.
   void NotifySurfaceIdAvailable(const SurfaceId& surface_id);
 
+  // Determines whether we should be observing the BeginFrameSource and
+  // starts/stops observing as necessary.
+  void UpdateNeedsBeginFrames();
+
+  // Acknowledge and confirm the last BeginFrame we received after we have
+  // activated all previously pending frames.
+  void AcknowledgeLastBeginFrame();
+
   SurfaceManager* const surface_manager_;
 
   // The last begin frame args generated by the begin frame source.
   BeginFrameArgs last_begin_frame_args_;
 
   // The BeginFrameSource used to set deadlines.
   BeginFrameSource* const begin_frame_source_;
 
   // The number of BeginFrames observed since a deadline was set. If
   // base::nullopt_t then a deadline is not set.
   base::Optional<uint32_t> frames_since_deadline_set_;
 
   // A map from a SurfaceId to the set of Surfaces blocked on that SurfaceId.
   std::unordered_map<SurfaceId, PendingSurfaceSet, SurfaceIdHash>
       blocked_surfaces_;
 
   PendingSurfaceSet pending_surfaces_;
 
+  // Whether we are currently observing the BeginFrameSource.
+  bool needs_begin_frames_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(SurfaceDependencyTracker);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_DEPENDENCY_TRACKER_H_