Use std::cbrt instead of our own CubeRoot implementation.

net/quic/congestion_control/cubic.cc

Index: net/quic/congestion_control/cubic.cc
diff --git a/net/quic/congestion_control/cubic.cc b/net/quic/congestion_control/cubic.cc
index 55ce8037cedb0c8ede254df8023f89286814f61a..fc4104e27bf47bcbbeb8159b3af6157626f32f63 100644
--- a/net/quic/congestion_control/cubic.cc
+++ b/net/quic/congestion_control/cubic.cc
@@ -10,6 +10,7 @@
 #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;
@@ -80,7 +81,6 @@ void Cubic::Reset() {
 
 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) {
@@ -135,8 +135,17 @@ QuicPacketCount Cubic::CongestionWindowAfterAck(
       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_ =
+            std::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_;
     }