Store ref in field trial in child processes

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 509ace4f77df5a6ec62a80d13eb18f19813a482b..ed5a3985998235cccc1a0572afb24957d6368374 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -596,9 +596,10 @@ class BASE_EXPORT FieldTrialList {
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
 
-  // Allocator used to instantiate field trial in child processes. In the
-  // future, we may want to move this to a more generic place if we want to
-  // start passing more data other than field trials.
+  // Allocator in shared memory containing field trial data. Used in both
+  // browser and child processes, but readonly in the child.
+  // In the future, we may want to move this to a more generic place if we want
+  // to start passing more data other than field trials.
   std::unique_ptr<SharedPersistentMemoryAllocator> field_trial_allocator_ =
       nullptr;