Store field trial parameters in shared memory

base/metrics/field_trial_param_associator.h

Index: base/metrics/field_trial_param_associator.h
diff --git a/base/metrics/field_trial_param_associator.h b/base/metrics/field_trial_param_associator.h
index 214e146966eebbdf8fe0888f4c705eea1f251fb0..72bb89743fa0277fb04b0d7e1241294b79192f3c 100644
--- a/base/metrics/field_trial_param_associator.h
+++ b/base/metrics/field_trial_param_associator.h
@@ -33,10 +33,17 @@ class BASE_EXPORT FieldTrialParamAssociator {
                                  const std::string& group_name,
                                  const FieldTrialParams& params);
 
-  // Gets the parameters for a field trial and its chosen group.
+  // Gets the parameters for a field trial. If the parameters are not in the
+  // mapping, then check the field trial entry in shared memory for the
+  // parameters and cache them here.
   bool GetFieldTrialParams(const std::string& trial_name,
                            FieldTrialParams* params);
 
+  // Same as above.
+  bool GetFieldTrialParams(const std::string& trial_name,

[Alexei Svitkine (slow), 2016/11/01 19:13:27]

Overloads are discouraged by the style guide. Change the name - e.g. add a
ByGroupName suffix. Also expand comment since it's not quite the same as above.

One important thing to mention is this version of the API doesn't activate the
trial. This is actually what we want for this use case - since serializing to
child processes shouldn't activate the trial. But it's important to explicitly
note this in the comment.

[lawrencewu, 2016/11/01 19:59:58]

Expanded the comment and changed the function to
GetFieldTrialParamsWithGroupName().

+                           const std::string& group_name,
+                           FieldTrialParams* params);
+
   // Clears the internal field_trial_params_ mapping.
   void ClearAllParamsForTesting();
 
@@ -46,6 +53,17 @@ class BASE_EXPORT FieldTrialParamAssociator {
   // (field_trial_name, field_trial_group)
   typedef std::pair<std::string, std::string> FieldTrialKey;
 
+  // Looks up the field trial entry in shared memory and gets the params from
+  // there.
+  bool GetParamsFromSharedMemory(const std::string& trial_name,
+                                 const std::string& group_name,
+                                 FieldTrialParams* params);
+
+  // Sets the parameters for |trial_name| and |group_name|. Caller must lock.
+  bool AssociateParamsWhileLocked(const std::string& trial_name,
+                                  const std::string& group_name,
+                                  const FieldTrialParams& params);
+
   Lock lock_;
   std::map<FieldTrialKey, FieldTrialParams> field_trial_params_;