Allow at most one preconnect to HTTP2 proxy servers

net/http/http_stream_factory_impl_job.cc

Index: net/http/http_stream_factory_impl_job.cc
diff --git a/net/http/http_stream_factory_impl_job.cc b/net/http/http_stream_factory_impl_job.cc
index 24ddae3a08d724f67b624697bbfc911c487128ca..375a71b9d3c5d208a7f573674fb410f542ac1141 100644
--- a/net/http/http_stream_factory_impl_job.cc
+++ b/net/http/http_stream_factory_impl_job.cc
@@ -476,6 +476,8 @@ void HttpStreamFactoryImpl::Job::OnHttpsProxyTunnelResponseCallback(
 }
 
 void HttpStreamFactoryImpl::Job::OnPreconnectsComplete() {
+  DCHECK(!new_spdy_session_);
+
   if (new_spdy_session_.get()) {
     delegate_->OnNewSpdySessionReady(this, new_spdy_session_,
                                      spdy_session_direct_);
@@ -805,6 +807,11 @@ int HttpStreamFactoryImpl::Job::DoInitConnectionImpl() {
   DCHECK(proxy_info_.proxy_server().is_valid());
   next_state_ = STATE_INIT_CONNECTION_COMPLETE;
 
+  if (delegate_->OnInitConnection(proxy_info_)) {
+    // Return since the connection initialization can be skipped.
+    return OK;
+  }
+
   using_ssl_ = origin_url_.SchemeIs(url::kHttpsScheme) ||
                origin_url_.SchemeIs(url::kWssScheme);
   using_spdy_ = false;