Let alr dectector use a budged to detect underuse.

webrtc/modules/pacing/interval_budget.h

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_MODULES_PACING_INTERVAL_BUDGET_H_
+#define WEBRTC_MODULES_PACING_INTERVAL_BUDGET_H_
+
+#include "webrtc/common_types.h"
+
+namespace webrtc {
+
+class IntervalBudget {
+ public:
+  explicit IntervalBudget(int initial_target_rate_kbps);
+  IntervalBudget(int initial_target_rate_kbps, bool can_build_up_underuse);
+  void set_target_rate_kbps(int target_rate_kbps);
+  void IncreaseBudget(int64_t delta_time_ms);
+  void UseBudget(size_t bytes);
+  size_t bytes_remaining() const;
+  int budget_level_percent() const;
+  int target_rate_kbps() const;
+
+ private:
+  int target_rate_kbps_;
+  int max_bytes_in_budget_;
+  int bytes_remaining_;
+  bool can_build_up_underuse_;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_PACING_INTERVAL_BUDGET_H_