[net] Add AsyncDns field trial.

chrome/browser/io_thread.cc

Index: chrome/browser/io_thread.cc
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index 7663c8a7418b60268085214e8950c1d9059049ff..b44263794d98e15d388d9a195409828b19bb3b43 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -21,6 +21,7 @@
 #include "build/build_config.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/extensions/event_router_forwarder.h"
+#include "chrome/browser/net/async_dns_field_trial.h"
 #include "chrome/browser/net/chrome_net_log.h"
 #include "chrome/browser/net/chrome_network_delegate.h"
 #include "chrome/browser/net/chrome_url_request_context.h"
@@ -109,10 +110,13 @@ class SystemURLRequestContext : public URLRequestContextWithUserAgent {
 net::HostResolver* CreateGlobalHostResolver(net::NetLog* net_log) {
   const CommandLine& command_line = *CommandLine::ForCurrentProcess();
 
+  bool allow_async_dns_field_trial = true;
+
   size_t parallelism = net::HostResolver::kDefaultParallelism;
 
   // Use the concurrency override from the command-line, if any.
   if (command_line.HasSwitch(switches::kHostResolverParallelism)) {
+    allow_async_dns_field_trial = false;
     std::string s =
         command_line.GetSwitchValueASCII(switches::kHostResolverParallelism);
 
@@ -129,6 +133,7 @@ net::HostResolver* CreateGlobalHostResolver(net::NetLog* net_log) {
 
   // Use the retry attempts override from the command-line, if any.
   if (command_line.HasSwitch(switches::kHostResolverRetryAttempts)) {
+    allow_async_dns_field_trial = false;
     std::string s =
         command_line.GetSwitchValueASCII(switches::kHostResolverRetryAttempts);
     // Parse the switch (it should be a non-negative integer).
@@ -141,14 +146,24 @@ net::HostResolver* CreateGlobalHostResolver(net::NetLog* net_log) {
   }
 
   net::HostResolver* global_host_resolver = NULL;
+  bool use_async = false;
   if (command_line.HasSwitch(switches::kEnableAsyncDns)) {
-    global_host_resolver =
-        net::CreateAsyncHostResolver(parallelism, retry_attempts, net_log);
+    allow_async_dns_field_trial = false;
+    use_async = true;
+  } else if (command_line.HasSwitch(switches::kDisableAsyncDns)) {
+    allow_async_dns_field_trial = false;
+    use_async = false;
   }
 
-  if (!global_host_resolver) {
+  if (allow_async_dns_field_trial)
+    use_async = chrome_browser_net::ConfigureAsyncDnsFieldTrial();
+
+  if (use_async) {
+    global_host_resolver =
+        net::CreateAsyncHostResolver(parallelism, retry_attempts, net_log);
+  } else {
     global_host_resolver =
-      net::CreateSystemHostResolver(parallelism, retry_attempts, net_log);
+        net::CreateSystemHostResolver(parallelism, retry_attempts, net_log);
   }
 
   // Determine if we should disable IPv6 support.