Server push cancellation: add a finch trial parameter

net/http/http_network_session.cc

Index: net/http/http_network_session.cc
diff --git a/net/http/http_network_session.cc b/net/http/http_network_session.cc
index fda084eab56900bcf246229fa108e0e9c72f91f0..d1fa02c77bb7ada11e5301866ce408078658ad3d 100644
--- a/net/http/http_network_session.cc
+++ b/net/http/http_network_session.cc
@@ -103,6 +103,7 @@ HttpNetworkSession::Params::Params()
     : client_socket_factory(nullptr),
       host_resolver(nullptr),
       cert_verifier(nullptr),
+      enable_server_push_cancellation(false),
       channel_id_service(nullptr),
       transport_security_state(nullptr),
       cert_transparency_verifier(nullptr),
@@ -172,6 +173,7 @@ HttpNetworkSession::HttpNetworkSession(const Params& params)
     : net_log_(params.net_log),
       http_server_properties_(params.http_server_properties),
       cert_verifier_(params.cert_verifier),
+      enable_server_push_cancellation_(params.enable_server_push_cancellation),
       http_auth_handler_factory_(params.http_auth_handler_factory),
       proxy_service_(params.proxy_service),
       ssl_config_service_(params.ssl_config_service),
@@ -396,7 +398,9 @@ bool HttpNetworkSession::IsProtocolEnabled(NextProto protocol) const {
 
 void HttpNetworkSession::SetServerPushDelegate(
     std::unique_ptr<ServerPushDelegate> push_delegate) {
-  DCHECK(!push_delegate_ && push_delegate);
+  DCHECK(push_delegate);
+  if (!enable_server_push_cancellation_ || push_delegate_)
+    return;
 
   push_delegate_ = std::move(push_delegate);
   spdy_session_pool_.set_server_push_delegate(push_delegate_.get());