Create A/B test of SDCH...

chrome/browser/browser_main.cc

Index: chrome/browser/browser_main.cc
===================================================================
--- chrome/browser/browser_main.cc	(revision 19579)
+++ chrome/browser/browser_main.cc	(working copy)
@@ -690,16 +690,35 @@
   PluginService::GetInstance()->SetChromePluginDataDir(profile->GetPath());
 
   // Prepare for memory caching of SDCH dictionaries.
-  SdchManager sdch_manager;  // Construct singleton database.
-  sdch_manager.set_sdch_fetcher(new SdchDictionaryFetcher);
-  // Use default of "" so that all domains are supported.
+  // Perform A/B test to measure global impact of SDCH support.
+  // Set up a field trial to see what disabling SDCH does to latency of page
+  // layout globally.
+  FieldTrial::Probability kSDCH_DIVISOR = 100;
+  FieldTrial::Probability kSDCH_PROBABILITY_PER_GROUP = 50;  // 50% probability.
+  scoped_refptr<FieldTrial> sdch_trial =
+      new FieldTrial("GlobalSdch", kSDCH_DIVISOR);
+
+  bool need_to_init_sdch = true;
   std::string switch_domain("");
   if (parsed_command_line.HasSwitch(switches::kSdchFilter)) {
     switch_domain =
         WideToASCII(parsed_command_line.GetSwitchValue(switches::kSdchFilter));
+  } else {
+    sdch_trial->AppendGroup("_global_disable_sdch",
+                            kSDCH_PROBABILITY_PER_GROUP);
+    int sdch_enabled = sdch_trial->AppendGroup("_global_enable_sdch",
+                                               kSDCH_PROBABILITY_PER_GROUP);
+    need_to_init_sdch = (sdch_enabled == sdch_trial->group());
   }
-  sdch_manager.EnableSdchSupport(switch_domain);
 
+  scoped_ptr<SdchManager> sdch_manager;  // Singleton database.
+  if (need_to_init_sdch) {
+    sdch_manager.reset(new SdchManager);
+    sdch_manager->set_sdch_fetcher(new SdchDictionaryFetcher);
+    // Use default of "" so that all domains are supported.
+    sdch_manager->EnableSdchSupport(switch_domain);
+  }
+
   MetricsService* metrics = NULL;
   if (!parsed_command_line.HasSwitch(switches::kDisableMetrics)) {
     bool enabled = local_state->GetBoolean(prefs::kMetricsReportingEnabled);