QUIC - add UMA histogram to track retransmit rate for large uploads.

R=asvitkine@chromium.org, rch@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2342663003
Cr-Commit-Position: refs/heads/master@{#418719}

net/quic/chromium/quic_chromium_client_session.cc

@@ -371,6 +371,13 @@ QuicChromiumClientSession::~QuicChromiumClientSession() {
   UMA_HISTOGRAM_COUNTS("Net.QuicSession.MtuProbesSent",
                        connection()->mtu_probe_count());
 
+  if (stats.packets_sent >= 100) {
+    // Used to monitor for regressions that effect large uploads.
+    UMA_HISTOGRAM_COUNTS_1000(
+        "Net.QuicSession.PacketRetransmitsPerMille",
+        1000 * stats.packets_retransmitted / stats.packets_sent);
+  }
+
   if (stats.max_sequence_reordering == 0)
     return;
   const base::HistogramBase::Sample kMaxReordering = 100;

tools/metrics/histograms/histograms.xml

@@ -31478,6 +31478,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.PacketRetransmitsPerMille" units="permille">
+  <owner>ckrasic@chromium.org</owner>
+  <summary>
+    The number of packets retransmitted per 1000.  Only sessions with
+    packets_sent &gt;= 100 are included.  Recorded in session destructor.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.PacketWriteTime">
   <owner>rch@chromium.org</owner>
   <summary>