[Not for review] Add TRACE_EVENTs and chrome://flags for preconnect

net/dns/dns_transaction.cc

Index: net/dns/dns_transaction.cc
diff --git a/net/dns/dns_transaction.cc b/net/dns/dns_transaction.cc
index 8c7a757724096773e40e90d5178c981f9cdfaee9..583e2b3a4c3d7bfb271c38580ca803441d9de050 100644
--- a/net/dns/dns_transaction.cc
+++ b/net/dns/dns_transaction.cc
@@ -26,6 +26,7 @@
 #include "base/threading/non_thread_safe.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/timer/timer.h"
+#include "base/trace_event/trace_event.h"
 #include "base/values.h"
 #include "net/base/completion_callback.h"
 #include "net/base/io_buffer.h"
@@ -284,6 +285,8 @@ class DnsUDPAttempt : public DnsAttempt {
   }
 
   void OnIOComplete(int rv) {
+    TRACE_EVENT1("net", "DnsUDPAttempt::OnIOComplete", "qname",
+                 query_->qname().as_string());
     rv = DoLoop(rv);
     if (rv != ERR_IO_PENDING)
       callback_.Run(rv);
@@ -801,6 +804,8 @@ class DnsTransactionImpl : public DnsTransaction,
   // Begins query for the current name. Makes the first attempt.
   AttemptResult StartQuery() {
     std::string dotted_qname = DNSDomainToString(qnames_.front());
+    TRACE_EVENT1("net", "DnsTransactionImpl::StartQuery", "dotted_qname",
+                 dotted_qname);
     net_log_.BeginEvent(NetLogEventType::DNS_TRANSACTION_QUERY,
                         NetLog::StringCallback("qname", &dotted_qname));