Support add-on 'field trial' records.

components/variations/active_field_trials.cc

Index: components/variations/active_field_trials.cc
diff --git a/components/variations/active_field_trials.cc b/components/variations/active_field_trials.cc
index 71ff95d7e9f126defc1c3d4deb1c7058a30b37a2..bf35bc060cbc2ae4fe26ceba7098b2cef7a3ed1a 100644
--- a/components/variations/active_field_trials.cc
+++ b/components/variations/active_field_trials.cc
@@ -40,8 +40,8 @@ void AppendActiveGroupIdsAsStrings(
 
 }  // namespace
 
-ActiveGroupId MakeActiveGroupId(const std::string& trial_name,
-                                const std::string& group_name) {
+ActiveGroupId MakeActiveGroupId(base::StringPiece trial_name,
+                                base::StringPiece group_name) {
   ActiveGroupId id;
   id.name = metrics::HashName(trial_name);
   id.group = metrics::HashName(group_name);