Refactor NACK bitrate allocation

webrtc/base/rate_statistics.cc

Index: webrtc/base/rate_statistics.cc
diff --git a/webrtc/base/rate_statistics.cc b/webrtc/base/rate_statistics.cc
index 1fd63cc6d2a071251edb0a9f4f3744fd700571f2..a4da58bf676343f9a68153641d9ba98a3b11d92d 100644
--- a/webrtc/base/rate_statistics.cc
+++ b/webrtc/base/rate_statistics.cc
@@ -61,8 +61,8 @@ void RateStatistics::Update(size_t count, int64_t now_ms) {
   ++num_samples_;
 }
 
-rtc::Optional<uint32_t> RateStatistics::Rate(int64_t now_ms) {
-  EraseOld(now_ms);
+rtc::Optional<uint32_t> RateStatistics::Rate(int64_t now_ms) const {
+  const_cast<RateStatistics*>(this)->EraseOld(now_ms);
 
   // If window is a single bucket or there is only one sample in a data set that
   // has not grown to the full window size, treat this as rate unavailable.
@@ -112,7 +112,7 @@ bool RateStatistics::SetWindowSize(int64_t window_size_ms, int64_t now_ms) {
   return true;
 }
 
-bool RateStatistics::IsInitialized() {
+bool RateStatistics::IsInitialized() const {
   return oldest_time_ != -max_window_size_ms_;
 }