Change FieldTrial::State to not use StringPiece members.

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 5c5240c42af6bc4ba773f1c2d1fdec21e1179ce1..c59036390ef51b630e3482e9a9fbee7d316a8b66 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -124,10 +124,12 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
   };
 
   // A triplet representing a FieldTrial, its selected group and whether it's
-  // active.
+  // active. String members are pointers to the underlying strings owned by the
+  // FieldTrial object. Does not use StringPiece to avoid conversions back to
+  // std::string.
   struct BASE_EXPORT State {
-    StringPiece trial_name;
-    StringPiece group_name;
+    const std::string* trial_name;

[brucedawson, 2017/01/24 00:44:14]

Consider doing "= nullptr;" here to make it crystal clear that an uninitialized
pointer is impossible. At that point you might as well also initialize
'activated', and then the constructor in the .cc file will just be empty.

[Alexei Svitkine (slow), 2017/01/24 16:40:19]

Done.

+    const std::string* group_name;
     bool activated;
 
     State();