Store field trial parameters in shared memory

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 98361732994ddb8bc35368d420c5edc7d11acb7a..7bdd2dd1df8bae653a8cf5db1d4da6366f828471 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -226,6 +226,8 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
 
   friend class RefCounted<FieldTrial>;
 
+  friend class FieldTrialParamAssociator;
+
   // This is the group number of the 'default' group when a choice wasn't forced
   // by a call to FieldTrialList::CreateFieldTrial. It is kept private so that
   // consumers don't use it by mistake in cases where the group was forced.
@@ -433,6 +435,15 @@ class BASE_EXPORT FieldTrialList {
   // metrics, crashes, etc.
   static std::string FindFullName(const std::string& trial_name);
 
+  // Looks in the allocator for |field_trial|'s params via its ref_, and stores
+  // it in |params|.

[Alexei Svitkine (slow), 2016/11/23 17:23:39]

This is talking about implementation detail.

Please change this comment to talk about what it does without implementation
detail - since it's a public function.

Also, move these new functions to the end of the list of public functions - they
shouldn't be in the middle of commonly used ones that are expected to be used by
many callers.

[lawrencewu, 2016/11/23 19:07:33]

Done.

+  static bool GetParamsFromSharedMemory(
+      FieldTrial* field_trial,
+      std::map<std::string, std::string>* params);
+
+  // Clears all the params in the allocator.
+  static void ClearParamsFromSharedMemoryForTesting();
+
   // Returns true if the named trial has been registered.
   static bool TrialExists(const std::string& trial_name);
 
@@ -544,6 +555,7 @@ class BASE_EXPORT FieldTrialList {
   // Allow tests to access our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, InstantiateAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AddTrialsToAllocator);
+  FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AssociateFieldTrialParams);
 
 #if defined(OS_WIN)
   // Takes in |handle| that should have been retrieved from the command line and
@@ -597,7 +609,7 @@ class BASE_EXPORT FieldTrialList {
   static bool used_without_global_;
 
   // Lock for access to registered_.
-  base::Lock lock_;
+  Lock lock_;
   RegistrationMap registered_;
 
   std::map<std::string, std::string> seen_states_;