Create A/B test of SDCH

To do this, I needed to add the feature that ALL FieldTrials that are
established in the browser process are forwarded and established in
the corresponding renderer processes.  This then allows both DNS impact,
 as well as SDCH inmpact (and any other field tests) to be studied
at the same time in a single binary.

This checkin also establishes a pattern that when we're doing A/B tests
via a histogram such as RequestToFinish, that we produce names for
all groups, rather than leaving one group as the "default" or "empty postfix"
group.  This is critical for naming various sub-groups when a multitude
of tests are taking place at the same time.

BUG=15479

r=mbelshe
Review URL: http://codereview.chromium.org/150087

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@19595 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 23 insertions, 4 deletions

diff --git a/chrome/browser/browser_main.cc b/chrome/browser/browser_main.cc
index 91bc5e4..5a52734 100644
--- a/chrome/browser/browser_main.cc
+++ b/chrome/browser/browser_main.cc
@@ -690,15 +690,34 @@ int BrowserMain(const MainFunctionParams& parameters) {
   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());
+  }
+
+  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);
   }
-  sdch_manager.EnableSdchSupport(switch_domain);
 
   MetricsService* metrics = NULL;
   if (!parsed_command_line.HasSwitch(switches::kDisableMetrics)) {