Activate prepopulated fieldtrials

chrome/browser/chrome_browser_main.cc

Index: chrome/browser/chrome_browser_main.cc
diff --git a/chrome/browser/chrome_browser_main.cc b/chrome/browser/chrome_browser_main.cc
index b1cfe0b6d89997f4e84d4136827d74d16974ed71..57eca16ffaebe8724d9601c792a1851d69b709fd 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -633,6 +633,12 @@ void ChromeBrowserMainParts::SetupMetricsAndFieldTrials() {
     CHECK(result) << "Invalid --" << switches::kForceFieldTrials
                   << " list specified.";
   }
+
+  if (command_line->HasSwitch(switches::kEnableFieldTrialTestingConfig)) {

[Alexei Svitkine (slow), 2015/07/10 15:24:32]

Didn't we want this to be on by default for Chromium builds?

Should we just not have a command-line flag and instead do this unconditionally
inside:

#if !defined(GOOGLE_CHROME_BUILD)

Or maybe I don't have the full context. How were you thinking that this would
work?

[danduong, 2015/07/10 16:24:20]

This is a good topic of discussion. My first thought is to just have it default
enabled on a non-official build, but disable-able from command-line. We'll also
have to be careful on how this effected by the other forcing flags for field
trials. I'm thinking if --force-field-trial is used, always disable this flag.

std::string enable_field_trial_testing =
    command_line->GetSwitchValueASCII(switches::kEnableFieldTrialTestingConfig);
if (enable_field_trial_testing.empty()) {
#if defined(GOOGLE_CHROME_BUILD) && defined(OFFICIAL_BUILD)
  enable_field_trial_testing = "0";
#else
  enable_field_trial_testing = "1";
#endif
}

if (enable_field_trial_testing == '1' &&
!command_line->HasSwitch(switches::kForceFieldTrials) {
 ...
}

[Alexei Svitkine (slow), 2015/07/10 19:05:12]

Default for unofficial builds but disable-able from the command-line SGTM. Have
you checked whether try bots pass this? (I'm hoping the default configs we have
don't cause any test failures, but it might indeed be the case. :\)

I think we shouldn't make it enable-able for official builds. We should just not
compile it in (i.e. so the whole block should be inside GOOGLE_CHROME_BUILD).
This will let the linker dead-code-eliminate the associated function / struct
and thus we wouldn't be paying any binary size penalty for this in official
builds.

+    bool result = chrome_variations::AssociateParamsFromFieldTrialConfig();
+    CHECK(result) << " Enabling field trial testing configuration failed.";
+  }
+
   if (command_line->HasSwitch(switches::kForceVariationIds)) {
     // Create default variation ids which will always be included in the
     // X-Client-Data request header.