Add other variations (with ids) for NTPSnippets to chrome://flags

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 1166a7f31c04fbf2c8ea7305a92023145c11118f..a6079bc819bc71c2fb836e6847edcbddad24d901 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -730,26 +730,36 @@ void ChromeBrowserMainParts::SetupMetricsAndFieldTrials() {
                   << " list specified.";
   }
 
+  std::unique_ptr<base::FeatureList> feature_list(new base::FeatureList);
+
+  // Associate parameters chosen in about:flags and create trial/group for them.
+  flags_ui::PrefServiceFlagsStorage flags_storage(
+      g_browser_process->local_state());
+  std::vector<std::string> variation_ids =
+      about_flags::RegisterAllFeatureVariationParameters(
+          &flags_storage, feature_list.get());
+
   if (command_line->HasSwitch(switches::kForceVariationIds)) {
+    // Combine the variation ids from chrome://flags with the ones from the
+    // command-line flag.
+    std::vector<std::string> variation_ids_flags = base::SplitString(
+        command_line->GetSwitchValueASCII(switches::kForceVariationIds), ",",
+        base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
+    variation_ids.insert(variation_ids.end(), variation_ids_flags.begin(),
+                         variation_ids_flags.end());
+  }

[Alexei Svitkine (slow), 2016/07/26 14:44:07]

I still think all this work should be being done in the variations code rather
than in chrome_browser_main.cc.

It's nice to make SetDefaultVariationIds(), but we should avoid bloating
chrome_browser_main.cc with extra variations code.

So, I still suggest putting this logic in the variations code. To keep it
simple, however, you can make a new function that does this parsing and
combining and then calls SetDefaultVariationIds(). This way,
SetDefaultVariationIds() can stay simple and the extra logic will be in the
wrapper function.

[jkrcal, 2016/07/26 15:50:39]

Done. It was not super clear to me where to cut the logic. Please let me know if
even more should be in variations (which would require moving the command-line
flag into variations::flags and/or adding metrics dependency into variations).

+
+  if (!variation_ids.empty()) {
     // 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.";
+    bool result = provider->SetDefaultVariationIds(variation_ids);
+    CHECK(result) << "Invalid list of variation ids specified (either in --"
+                  << switches::kForceVariationIds << " or in chrome://flags)";
     metrics->AddSyntheticTrialObserver(provider);
   }
 
-  std::unique_ptr<base::FeatureList> feature_list(new base::FeatureList);
-
-  // Associate parameters chosen in about:flags and create trial/group for them.
-  flags_ui::PrefServiceFlagsStorage flags_storage(
-      g_browser_process->local_state());
-  about_flags::RegisterAllFeatureVariationParameters(&flags_storage,
-    feature_list.get());
-
   feature_list->InitializeFromCommandLine(
       command_line->GetSwitchValueASCII(switches::kEnableFeatures),
       command_line->GetSwitchValueASCII(switches::kDisableFeatures));