Move EntropyProvider Source From MetricsService to MetricsServicesManager

ios/chrome/browser/ios_chrome_main_parts.mm

Index: ios/chrome/browser/ios_chrome_main_parts.mm
diff --git a/ios/chrome/browser/ios_chrome_main_parts.mm b/ios/chrome/browser/ios_chrome_main_parts.mm
index a3ede343d21b4ec794dc59a4e44c0c30acfe2006..92c75268608c53909ce3c389875fa15aeb70a499 100644
--- a/ios/chrome/browser/ios_chrome_main_parts.mm
+++ b/ios/chrome/browser/ios_chrome_main_parts.mm
@@ -193,15 +193,12 @@ void IOSChromeMainParts::SetUpMetricsAndFieldTrials() {
   base::SetRecordActionTaskRunner(
       web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
 
-  // 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).
-  metrics::MetricsService* metrics = application_context_->GetMetricsService();
-
   // Initialize FieldTrialList to support FieldTrials that use one-time
   // randomization.

[Alexei Svitkine (slow), 2016/09/14 20:26:37]

Can you add the DCHECK() that's in the non-iOS code here too, for consistency?
Thanks!

[robliao, 2016/09/14 20:54:42]

Done.

   field_trial_list_.reset(
-      new base::FieldTrialList(metrics->CreateEntropyProvider().release()));
+      new base::FieldTrialList(application_context_->GetMetricsServicesManager()
+                                   ->CreateEntropyProvider()
+                                   .release()));
 
   const base::CommandLine* command_line =
       base::CommandLine::ForCurrentProcess();
@@ -225,6 +222,11 @@ void IOSChromeMainParts::SetUpMetricsAndFieldTrials() {
   std::vector<std::string> variation_ids =
       RegisterAllFeatureVariationParameters(&flags_storage, feature_list.get());
 
+  // Must initialize metrics after labs have been converted into switches,

[Alexei Svitkine (slow), 2016/09/14 20:26:38]

Nit: "labs" terminology is pretty outdated. Given you're moving this, change
"labs" to "about:flags". Here and in the non-iOS one. In the non-iOS code move
the comment to the same place as here, please.

[robliao, 2016/09/14 20:54:42]

Done.

+  // but before field trials are set up (so that client ID is available for
+  // one-time randomized field trials).
+  metrics::MetricsService* metrics = application_context_->GetMetricsService();
+

[Alexei Svitkine (slow), 2016/09/14 20:26:38]

Nit: Make it consistent with the non-iOS code - either an empty line on both or
no empty line in both. (I don't have a preference either way).

[robliao, 2016/09/14 20:54:42]

This was lost in a file I forgot to save! It's there now.

   variations::VariationsHttpHeaderProvider* http_header_provider =
       variations::VariationsHttpHeaderProvider::GetInstance();
   // Force the variation ids selected in chrome://flags and/or specified using