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

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index b5dea9b3628c3f9a5fc347949651387e089684ac..1e6218a5f9141b222690d1aef622fd7b572e1250 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -381,8 +381,8 @@ FieldTrial* FieldTrialList::FactoryGetFieldTrial(
     FieldTrial::RandomizationType randomization_type,
     int* default_group_number) {
   return FactoryGetFieldTrialWithRandomizationSeed(
-      trial_name, total_probability, default_group_name,
-      year, month, day_of_month, randomization_type, 0, default_group_number);
+      trial_name, total_probability, default_group_name, year, month,
+      day_of_month, randomization_type, 0, default_group_number, NULL);
 }
 
 // static
@@ -395,7 +395,8 @@ FieldTrial* FieldTrialList::FactoryGetFieldTrialWithRandomizationSeed(
     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) {
   if (default_group_number)
     *default_group_number = FieldTrial::kDefaultGroupNumber;
   // Check if the field trial has already been created in some other way.
@@ -429,8 +430,10 @@ FieldTrial* FieldTrialList::FactoryGetFieldTrialWithRandomizationSeed(
 
   double entropy_value;
   if (randomization_type == FieldTrial::ONE_TIME_RANDOMIZED) {
+    // If an override entropy provider is given, use it.
     const FieldTrial::EntropyProvider* entropy_provider =
-        GetEntropyProviderForOneTimeRandomization();
+        override_entropy_provider ? override_entropy_provider
+                                  : GetEntropyProviderForOneTimeRandomization();
     CHECK(entropy_provider);
     entropy_value = entropy_provider->GetEntropyForTrial(trial_name,
                                                          randomization_seed);