Add one-time randomization support for FieldTrial, and the ability to

chrome/browser/browser_main.cc

Index: chrome/browser/browser_main.cc
diff --git a/chrome/browser/browser_main.cc b/chrome/browser/browser_main.cc
index e8c9724b6e25e6e0dcfbe138ed8572e0603aacc5..f06f716442f332fd3be7ce998aa4b9f248b2396d 100644
--- a/chrome/browser/browser_main.cc
+++ b/chrome/browser/browser_main.cc
@@ -1324,8 +1324,20 @@ int BrowserMain(const MainFunctionParams& parameters) {
   about_flags::ConvertFlagsToSwitches(local_state,
                                       CommandLine::ForCurrentProcess());
 
-  // Now the command line has been mutated based on about:flags, we can run some
-  // field trials
+  // Must initialize metrics after labs have been converted into switches,
+  // but before field trials are set up (so that client ID is available for
+  // one-time randomized field trials).
+  MetricsService* metrics = InitializeMetrics(parsed_command_line, local_state);

[jar (doing other things), 2011/04/29 00:57:11]

This will probably work.  I'm always nervous about moving much around at startup
time.  There is always the sad chance that we'll change dll load ordering or
such.  Be sure to watch the startup perf dashboard as you land with this change.

[Jói, 2011/05/02 18:55:03]

I will watch the startup perf dash.

[Jói, 2011/05/05 15:07:39]

Just to confirm, I looked in detail at every Startup/New Tab Cold/New Tab Warm
perf chart and there is nothing to indicate this change slowed things down.  I
also glanced over the rest of the perf dashboard, again no indication of a
slowdown.

Cheers,
Jói

+
+  // Initialize FieldTrialList to support FieldTrials that use one-time
+  // randomization.  The client ID will be empty if the user has not opted
+  // to send metrics.
+  std::string client_id = metrics->GetClientId();
+  if (!client_id.empty())
+    base::FieldTrialList::EnableOneTimeRandomization(client_id);
+
+  // Now the command line has been mutated based on about:flags, and one-time
+  // randomization has been enabled, we can run some field trials
   parts->SetupFieldTrials();

[jar (doing other things), 2011/04/29 00:57:11]

Wouldn't lines 1327-1337 fit nicely into SetupFieldTrials()?

Note that this function always grows bigger and bigger... and folks keep trying
to factor out sections.  It sure seems like the initialization should fit nicely
in there.

[Jói, 2011/05/02 18:55:03]

Good point.  I've factored it out.

 
   // Now that all preferences have been registered, set the install date
@@ -1670,7 +1682,6 @@ int BrowserMain(const MainFunctionParams& parameters) {
   sdch_manager.set_sdch_fetcher(new SdchDictionaryFetcher);
   sdch_manager.EnableSdchSupport(sdch_supported_domain);
 
-  MetricsService* metrics = InitializeMetrics(parsed_command_line, local_state);
   InstallJankometer(parsed_command_line);
 
 #if defined(OS_WIN) && !defined(GOOGLE_CHROME_BUILD)