Add Alternative Service field trial.

Before this CL:
 * If "use_alternative_service" QUIC trial param was set, both same host and
   different host Alt-Svc headers were parsed and used.  If this param was not
   set, no Alt-Svc headers were parsed, and in-memory entries (from
   Alternate-protocol headers or loaded from disk) were only used if they
   referred to the same host.

After this CL:
 * AltSvc field trial controls whether Alt-Svc headers are parsed.
 * "enable_alternative_service_with_different_host" QUIC trial param control
   whether in-memory Alt-Svc entries with different host (parsed in current
   session or loaded from disk) are used.

Default values are not changed by this CL.

This CL allows testing of same-host Alt-Svc header behavior.  If no issues are
encountered, parsing Alt-Svc headers and using same-host entries can be enabled
by default, and Alternate-Protocols removed.

BUG=581331

Review URL: https://codereview.chromium.org/1639913002

Cr-Commit-Position: refs/heads/master@{#372348}
diff --git a/net/quic/quic_network_transaction_unittest.cc b/net/quic/quic_network_transaction_unittest.cc
index d6f5c3c..f161588 100644
--- a/net/quic/quic_network_transaction_unittest.cc
+++ b/net/quic/quic_network_transaction_unittest.cc
@@ -442,7 +442,8 @@
     test_network_quality_estimator_->AddRTTObserver(&rtt_observer_);
 
     if (use_next_protos) {
-      params_.use_alternative_services = true;
+      params_.parse_alternative_services = true;
+      params_.enable_alternative_service_with_different_host = true;
       params_.next_protos = NextProtosWithSpdyAndQuic(true, true);
     }