Update from https://crrev.com/308996

net/quic/congestion_control/cubic.cc

 // Copyright (c) 2012 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.
 
 #include "net/quic/congestion_control/cubic.h"
 
+#include <math.h>
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/logging.h"
 #include "base/time/time.h"
 #include "net/quic/congestion_control/cube_root.h"
+#include "net/quic/quic_flags.h"
 #include "net/quic/quic_protocol.h"
 
 using std::max;
 
 namespace net {
 
 namespace {
 
 // Constants based on TCP defaults.
 // The following constants are in 2^10 fractions of a second instead of ms to
@@ skipping 50 matching lines @@
   last_max_congestion_window_ = 0;
   acked_packets_count_ = 0;
   estimated_tcp_congestion_window_ = 0;
   origin_point_congestion_window_ = 0;
   time_to_origin_point_ = 0;
   last_target_congestion_window_ = 0;
 }
 
 void Cubic::UpdateCongestionControlStats(QuicPacketCount new_cubic_mode_cwnd,
                                          QuicPacketCount new_reno_mode_cwnd) {
-
   QuicPacketCount highest_new_cwnd = max(new_cubic_mode_cwnd,
                                          new_reno_mode_cwnd);
   if (last_congestion_window_ < highest_new_cwnd) {
     // cwnd will increase to highest_new_cwnd.
     stats_->cwnd_increase_congestion_avoidance +=
         highest_new_cwnd - last_congestion_window_;
     if (new_cubic_mode_cwnd > new_reno_mode_cwnd) {
       // This cwnd increase is due to cubic mode.
       stats_->cwnd_increase_cubic_mode +=
           new_cubic_mode_cwnd - last_congestion_window_;
@@ skipping 34 matching lines @@
     // First ACK after a loss event.
     DVLOG(1) << "Start of epoch";
     epoch_ = current_time;  // Start of epoch.
     acked_packets_count_ = 1;  // Reset count.
     // Reset estimated_tcp_congestion_window_ to be in sync with cubic.
     estimated_tcp_congestion_window_ = current_congestion_window;
     if (last_max_congestion_window_ <= current_congestion_window) {
       time_to_origin_point_ = 0;
       origin_point_congestion_window_ = current_congestion_window;
     } else {
-      time_to_origin_point_ = CubeRoot::Root(kCubeFactor *
-          (last_max_congestion_window_ - current_congestion_window));
+      if (FLAGS_quic_use_std_cbrt) {
+        time_to_origin_point_ = static_cast<uint32>(
+            cbrt(kCubeFactor *
+                 (last_max_congestion_window_ - current_congestion_window)));
+      } else {
+        // TODO(rjshade): Remove CubeRoot source when removing
+        // FLAGS_quic_use_std_cbrt.
+        time_to_origin_point_ =
+            CubeRoot::Root(kCubeFactor * (last_max_congestion_window_ -
+                                          current_congestion_window));
+      }
       origin_point_congestion_window_ =
           last_max_congestion_window_;
     }
   }
   // Change the time unit from microseconds to 2^10 fractions per second. Take
   // the round trip time in account. This is done to allow us to use shift as a
   // divide operator.
   int64 elapsed_time =
       (current_time.Add(delay_min).Subtract(epoch_).ToMicroseconds() << 10) /
       base::Time::kMicrosecondsPerSecond;
@@ skipping 31 matching lines @@
   // congestion_window, use highest (fastest).
   if (target_congestion_window < estimated_tcp_congestion_window_) {
     target_congestion_window = estimated_tcp_congestion_window_;
   }
 
   DVLOG(1) << "Target congestion_window: " << target_congestion_window;
   return target_congestion_window;
 }
 
 }  // namespace net