Activate prepopulated fieldtrials

chrome/browser/chrome_browser_main.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chrome_browser_main.h"
 
 #include <set>
 #include <string>
 #include <vector>
 
@@ skipping 615 matching lines @@
 
     // Create field trials without activating them, so that this behaves in a
     // consistent manner with field trials created from the server.
     bool result = base::FieldTrialList::CreateTrialsFromString(
         command_line->GetSwitchValueASCII(switches::kForceFieldTrials),
         base::FieldTrialList::DONT_ACTIVATE_TRIALS,
         unforceable_field_trials);
     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.
     variations::VariationsHttpHeaderProvider* provider =
         variations::VariationsHttpHeaderProvider::GetInstance();
     bool result = provider->SetDefaultVariationIds(
         command_line->GetSwitchValueASCII(switches::kForceVariationIds));
     CHECK(result) << "Invalid --" << switches::kForceVariationIds
                   << " list specified.";
     metrics->AddSyntheticTrialObserver(provider);
@@ skipping 1134 matching lines @@
   chromeos::CrosSettings::Shutdown();
 #endif  // defined(OS_CHROMEOS)
 #endif  // defined(OS_ANDROID)
 }
 
 // Public members:
 
 void ChromeBrowserMainParts::AddParts(ChromeBrowserMainExtraParts* parts) {
   chrome_extra_parts_.push_back(parts);
 }