SurfaceDependencyTracker: Only observe BeginFrames if a deadline is needed

cc/surfaces/surface_dependency_deadline.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_DEADLINE_H_
+#define CC_SURFACES_SURFACE_DEPENDENCY_DEADLINE_H_
+
+#include "cc/scheduler/begin_frame_source.h"
+
+namespace cc {
+
+class SurfaceDependencyTracker;
+
+class SurfaceDependencyDeadline : public BeginFrameObserver {
+ public:
+  SurfaceDependencyDeadline(SurfaceDependencyTracker* dependency_tracker,
+                            BeginFrameSource* begin_frame_source);
+  ~SurfaceDependencyDeadline() override;
+
+  void Set(uint32_t number_of_frames_to_deadline);
+  void Cancel();
+
+  bool has_deadline() const {
+    return number_of_frames_to_deadline_.has_value();
+  }
+
+  // BeginFrameObserver implementation.
+  void OnBeginFrame(const BeginFrameArgs& args) override;
+  const BeginFrameArgs& LastUsedBeginFrameArgs() const override;
+  void OnBeginFrameSourcePausedChanged(bool paused) override;
+
+ private:
+  SurfaceDependencyTracker* const dependency_tracker_;
+  BeginFrameSource* begin_frame_source_ = nullptr;
+  base::Optional<uint32_t> number_of_frames_to_deadline_;
+
+  BeginFrameArgs last_begin_frame_args_;
+
+  DISALLOW_COPY_AND_ASSIGN(SurfaceDependencyDeadline);
+};
+
+}  // namespace cc
+
+#endif  // CC_SURFACES_SURFACE_DEPENDENCY_DEADLINE_H_