Use low entropy for studies that send experiment IDs to Google properties.

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index 808c7789740c23eb8ef42b21647f140875778658..fc6237a51343ee7d7753727ef96ba4c96b5025d9 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -393,9 +393,12 @@ class BASE_EXPORT FieldTrialList {
   // used on one-time randomized field trials (instead of a hash of the trial
   // name, which is used otherwise or if |randomization_seed| has value 0). The
   // |randomization_seed| value (other than 0) should never be the same for two
-  // trials, else this would result in correlated group assignments.
-  // Note: Using a custom randomization seed is only supported by the
-  // PermutedEntropyProvider (which is used when UMA is not enabled).
+  // trials, else this would result in correlated group assignments.  Note:
+  // Using a custom randomization seed is only supported by the
+  // PermutedEntropyProvider (which is used when UMA is not enabled). If
+  // |override_entropy_provider| is not null, then it will be used for
+  // randomization instead of the provider given when the FieldTrialList was
+  // instanciated.
   static FieldTrial* FactoryGetFieldTrialWithRandomizationSeed(
       const std::string& trial_name,
       FieldTrial::Probability total_probability,
@@ -405,7 +408,8 @@ class BASE_EXPORT FieldTrialList {
       const int day_of_month,
       FieldTrial::RandomizationType randomization_type,
       uint32_t randomization_seed,
-      int* default_group_number);
+      int* default_group_number,
+      const FieldTrial::EntropyProvider* override_entropy_provider);
 
   // The Find() method can be used to test to see if a named trial was already
   // registered, or to retrieve a pointer to it from the global map.