Attempt to detect impact of disabling dns prefetching on transaction latency

This is also meant to exercise the field trial infrastructure.

r=mbelshe
Review URL: http://codereview.chromium.org/50084

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@12271 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 14 insertions, 4 deletions

diff --git a/chrome/browser/browser_main.cc b/chrome/browser/browser_main.cc
index 4309f9d..1ef1f74 100644
--- a/chrome/browser/browser_main.cc
+++ b/chrome/browser/browser_main.cc
@@ -439,10 +439,20 @@ int BrowserMain(const MainFunctionParams& parameters) {
   net::EnsureWinsockInit();
 #endif  // defined(OS_WIN)
 
-  // Initialize the DNS prefetch system
-  chrome_browser_net::DnsPrefetcherInit dns_prefetch_init(user_prefs);
-  chrome_browser_net::DnsPrefetchHostNamesAtStartup(user_prefs, local_state);
-  chrome_browser_net::RestoreSubresourceReferrers(local_state);
+  // Set up a field trial.
+  FieldTrial::Probability kDIVISOR = 100;
+  FieldTrial::Probability kDISABLE = 1;  // 1%.
+  scoped_refptr<FieldTrial> dns_trial = new FieldTrial("DnsImpact", kDIVISOR);
+  int disabled_group = dns_trial->AppendGroup("_disabled_prefetch", kDISABLE);
+
+  scoped_ptr<chrome_browser_net::DnsPrefetcherInit> dns_prefetch_init;
+  if (dns_trial->group() != disabled_group) {
+    // Initialize the DNS prefetch system
+    dns_prefetch_init.reset(
+        new chrome_browser_net::DnsPrefetcherInit(user_prefs));
+    chrome_browser_net::DnsPrefetchHostNamesAtStartup(user_prefs, local_state);
+    chrome_browser_net::RestoreSubresourceReferrers(local_state);
+  }
 
 #if defined(OS_WIN)
   // Init common control sex.