Use a different algorithm with the low entropy source for field trials.

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
===================================================================
--- base/metrics/field_trial.h	(revision 151948)
+++ base/metrics/field_trial.h	(working copy)
@@ -94,6 +94,18 @@
  public:
   typedef int Probability;  // Probability type for being selected in a trial.
 
+  // EntropyProvider is an interface for providing entropy for one-time
+  // randomized (persistent) field trials.
+  class BASE_EXPORT EntropyProvider {
+   public:
+    virtual ~EntropyProvider() {}
+
+    // Returns a double in the range of [0, 1) based on |trial_name| that will
+    // be used for the dice roll for the specified field trial. A given instance
+    // should always return the same value given the same input |trial_name|.
+    virtual double GetEntropyForTrial(const std::string& trial_name) = 0;
+  };
+
   // A pair representing a Field Trial and its selected group.
   struct SelectedGroup {
     std::string trial;
@@ -188,7 +200,8 @@
   // consumers don't use it by mistake in cases where the group was forced.
   static const int kDefaultGroupNumber;
 
-  FieldTrial(const std::string& name,
+  FieldTrial(EntropyProvider* entropy_provider,
+             const std::string& name,
              Probability total_probability,
              const std::string& default_group_name);
   virtual ~FieldTrial();
@@ -202,11 +215,9 @@
   // Returns the group_name. A winner need not have been chosen.
   std::string group_name_internal() const { return group_name_; }
 
-  // Calculates a uniformly-distributed double between [0.0, 1.0) given
-  // a |client_id| and a |trial_name| (the latter is used as salt to avoid
-  // separate one-time randomized trials from all having the same results).
-  static double HashClientId(const std::string& client_id,
-                             const std::string& trial_name);
+  // Entropy provider that will be used if the field trial uses one-time
+  // randomization. Weak pointer, owned by FieldTrialList.
+  EntropyProvider* entropy_provider_;
 
   // The name of the field trial, as can be found via the FieldTrialList.
   const std::string name_;
@@ -278,11 +289,11 @@
 
   // This singleton holds the global list of registered FieldTrials.
   //
-  // |client_id| should be an opaque, diverse ID for this client that does not
-  // change between sessions, to enable one-time randomized trials. The empty
-  // string may be provided, in which case one-time randomized trials will
-  // not be available.
-  explicit FieldTrialList(const std::string& client_id);
+  // To support one-time randomized field trials, specify a non-NULL
+  // |entropy_provider| which should be a source of uniformly distributed
+  // entropy values.
+  explicit FieldTrialList(FieldTrial::EntropyProvider* entropy_provider);
+
   // Destructor Release()'s references to all registered FieldTrial instances.
   ~FieldTrialList();
 
@@ -393,12 +404,6 @@
   // when constructing the FieldTrialList singleton.
   static bool IsOneTimeRandomizationEnabled();
 
-  // Returns an opaque, diverse ID for this client that does not change
-  // between sessions.
-  //
-  // Returns the empty string if one-time randomization is not enabled.
-  static const std::string& client_id();
-
  private:
   // A map from FieldTrial names to the actual instances.
   typedef std::map<std::string, FieldTrial*> RegistrationList;
@@ -428,9 +433,9 @@
   base::Lock lock_;
   RegistrationList registered_;
 
-  // An opaque, diverse ID for this client that does not change
-  // between sessions, or the empty string if not initialized.
-  std::string client_id_;
+  // Entropy provider to be used for one-time randomized field trials. If NULL,
+  // one-time randomization is not supported.
+  scoped_ptr<FieldTrial::EntropyProvider> entropy_provider_;
 
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;