Adding local field trial for metrics/crash reports sampling.

chrome/browser/metrics/chrome_metrics_services_manager_client.h

Index: chrome/browser/metrics/chrome_metrics_services_manager_client.h
diff --git a/chrome/browser/metrics/chrome_metrics_services_manager_client.h b/chrome/browser/metrics/chrome_metrics_services_manager_client.h
index 7106e1ff7843713b2a43680a9409e5e8667e7386..2cba423548fa66f9ac68f52922ac266da90fc9ad 100644
--- a/chrome/browser/metrics/chrome_metrics_services_manager_client.h
+++ b/chrome/browser/metrics/chrome_metrics_services_manager_client.h
@@ -27,6 +27,14 @@ class ChromeMetricsServicesManagerClient
   explicit ChromeMetricsServicesManagerClient(PrefService* local_state);
   ~ChromeMetricsServicesManagerClient() override;
 
+  // Unconditionally attempts to create a field trial to controll client side

[Alexei Svitkine (slow), 2016/08/30 18:17:05]

Nit: controll -> control

[jwd, 2016/08/30 18:59:41]

Done.

+  // metrics/crash sampling to use as a fallback when one hasn't been
+  // provided. This is expected to occur on first-run on platforms that don't
+  // have variations first-run support. This should only be called when there is

[Alexei Svitkine (slow), 2016/08/30 18:17:05]

Nit: "variations first-run" -> "first-run variations"

[jwd, 2016/08/30 18:59:41]

Done.

+  // no existing field trial controlling the sampling feature, and on the
+  // correct platform and channel.
+  static void CreateFallbackSamplingTrial(base::FeatureList* feature_list);
+
   // Determines if this client is eligible to send metrics. If they are, and
   // there was user consent, then metrics and crashes would be reported.
   static bool IsClientInSample();