Added a Unique ID for a Field Trial containing it's hashed name and the selected group ID.

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
===================================================================
--- base/metrics/field_trial.h	(revision 118842)
+++ base/metrics/field_trial.h	(working copy)
@@ -92,6 +92,12 @@
 class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
  public:
   typedef int Probability;  // Probability type for being selected in a trial.
+  // The Unique ID of a trial, where the name and group identifiers are
+  // hashes of the trial and group name strings.
+  struct NameGroupId{

[Alexei Svitkine (slow), 2012/01/25 15:25:19]

Nit: Add a space before {.

[MAD, 2012/01/25 17:02:33]

Done.

+    uint32 name;
+    uint32 group;
+  };
 
   // A return value to indicate that a given instance has not yet had a group
   // assignment (and hence is not yet participating in the trial).
@@ -101,6 +107,9 @@
   // to assign all the remaining probability to a group ('default').
   static const int kDefaultGroupNumber;
 
+  // This value is reserved for an uninitialized hash value.
+  static const uint32 kReservedHashValue;
+
   // The name is used to register the instance with the FieldTrialList class,
   // and can be used to find the trial (only one trial can be present for each
   // name).  |name| and |default_group_name| may not be empty.
@@ -148,6 +157,10 @@
   // number is used as the group name.
   std::string group_name();
 
+  // Gets the unique identifier of the Field Trial, but only if a winner was
+  // elected. Returns true if a winner is returned, false otherwise.

[Alexei Svitkine (slow), 2012/01/25 15:25:19]

Can you reword this to either explain what you mean by "winner", or using
different terms? Also "Returns true if a winner is returned" could be worded
better, even if you do wish to keep the term "winner".

[MAD, 2012/01/25 17:02:33]

Done.

+  bool GetNameGroupId(NameGroupId* name_group_id);
+
   // Return the default group name of the FieldTrial.
   std::string default_group_name() const { return default_group_name_; }
 
@@ -174,6 +187,7 @@
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, HashClientId);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, HashClientIdIsUniform);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, UseOneTimeRandomization);
+  FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, NameGroupIds);
 
   friend class base::FieldTrialList;
 
@@ -190,9 +204,15 @@
   static double HashClientId(const std::string& client_id,
                              const std::string& trial_name);
 
+  // Creates unique identifier for the trial by hashing the name string.

[Alexei Svitkine (slow), 2012/01/25 15:25:19]

The comment should probably mention that this hash function is used for hashing
both name and group name, not just the name.

[MAD, 2012/01/25 17:02:33]

Done.

+  static uint32 HashName(const std::string& name);
+
   // The name of the field trial, as can be found via the FieldTrialList.
   const std::string name_;
 
+  // The hashed name of the field trial to be sent as a unique identifier.
+  const uint32 name_hash_;
+
   // The maximum sum of all probabilities supplied, which corresponds to 100%.
   // This is the scaling factor used to adjust supplied probabilities.
   const Probability divisor_;
@@ -218,6 +238,10 @@
   // has been called.
   std::string group_name_;
 
+  // The hashed name of the group to be sent as a unique identifier.
+  // Is not valid while group_ is equal to kNotFinalized.
+  uint32 group_name_hash_;
+
   // When enable_field_trial_ is false, field trial reverts to the 'default'
   // group.
   bool enable_field_trial_;
@@ -285,11 +309,15 @@
 
   // Create a persistent representation of all FieldTrial instances and the
   // |client_id()| state for resurrection in another process.  This allows
-  // randomization to be done in one process, and secondary processes can by
+  // randomization to be done in one process, and secondary processes can be
   // synchronized on the result. The resulting string contains the
   // |client_id()|, the names, the trial name, and a "/" separator.
   static void StatesToString(std::string* output);
 
+  // Returns an array of Unique IDs for each currently running Field Trials.

[Alexei Svitkine (slow), 2012/01/25 15:25:19]

Can you expand this comment to explain what "currently running" means? e.g. that
if a group hasn't been chosen yet, then it is not included.

[MAD, 2012/01/25 17:02:33]

Done.

+  static void GetFieldTrialNameGroupIds(
+      std::vector<FieldTrial::NameGroupId>* name_group_ids);
+
   // Use a previously generated state string (re: StatesToString()) augment the
   // current list of field tests to include the supplied tests, and using a 100%
   // probability for each test, force them to have the same group string.  This