Change FieldTrial::State to not use StringPiece members.

base/metrics/field_trial.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 106 matching lines @@
                                       uint32_t randomization_seed) const = 0;
   };
 
   // A pair representing a Field Trial and its selected group.
   struct ActiveGroup {
     std::string trial_name;
     std::string group_name;
   };
 
   // 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();
     State(const State& other);
     ~State();
   };
 
   // We create one FieldTrialEntry per field trial in shared memory, via
   // AddToAllocatorWhileLocked. The FieldTrialEntry is followed by a
   // base::Pickle object that we unpickle and read from.
@@ skipping 590 matching lines @@
 
   // Tracks whether CreateTrialsFromCommandLine() has been called.
   bool create_trials_from_command_line_called_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_