Adds ability to disable connection pooling in QUIC.

BUG=

Review URL: https://codereview.chromium.org/551903002

Cr-Commit-Position: refs/heads/master@{#293823}
diff --git a/net/http/http_network_session.h b/net/http/http_network_session.h
index da8ac72a2..d1e4234 100644
--- a/net/http/http_network_session.h
+++ b/net/http/http_network_session.h
@@ -112,6 +112,7 @@
     bool enable_quic_port_selection;
     bool enable_quic_time_based_loss_detection;
     bool quic_always_require_handshake_confirmation;
+    bool quic_disable_connection_pooling;
     HostPortPair origin_to_force_quic_on;
     QuicClock* quic_clock;  // Will be owned by QuicStreamFactory.
     QuicRandom* quic_random;