Add histograms to track how long QUIC read/write operations take to complete.

net/quic/quic_client_session.cc

Index: net/quic/quic_client_session.cc
diff --git a/net/quic/quic_client_session.cc b/net/quic/quic_client_session.cc
index ef8bc202f1ca5caa3197306a9210d5fd406d3149..1a99cf94a2a82d5d8a2db485294a5598ab3b08d4 100644
--- a/net/quic/quic_client_session.cc
+++ b/net/quic/quic_client_session.cc
@@ -761,6 +761,7 @@ void QuicClientSession::StartReading() {
                          read_buffer_->size(),
                          base::Bind(&QuicClientSession::OnReadComplete,
                                     weak_factory_.GetWeakPtr()));
+  UMA_HISTOGRAM_BOOLEAN("Net.QuicSession.AsyncRead", rv == ERR_IO_PENDING);
   if (rv == ERR_IO_PENDING) {
     num_packets_read_ = 0;
     return;