Remove std::string histogram name from HistogramBase object.

extensions/browser/value_store/lazy_leveldb.h

 // Copyright 2016 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.
 
 #ifndef EXTENSIONS_BROWSER_VALUE_STORE_LAZY_LEVELDB_H_
 #define EXTENSIONS_BROWSER_VALUE_STORE_LAZY_LEVELDB_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 44 matching lines @@
   // be returned and value will be unchanged. Caller must ensure the database is
   // open before calling this method.
   ValueStore::Status Read(const std::string& key,
                           std::unique_ptr<base::Value>* value);
 
   // Opens the underlying database if not yet open. If the open fails due to
   // corruption will attempt to repair the database. Failing that, will attempt
   // to delete the database. Will only attempt a single recovery.
   ValueStore::Status EnsureDbIsOpen();
 
-  const std::string& open_histogram_name() const {
+  const char* open_histogram_name() const {
     return open_histogram_->histogram_name();
   }
 
   leveldb::DB* db() { return db_.get(); }
 
   const leveldb::ReadOptions& read_options() const { return read_options_; }
 
   const leveldb::WriteOptions& write_options() const { return write_options_; }
 
  private:
@@ skipping 15 matching lines @@
   bool db_unrecoverable_ = false;
   // Used for UMA logging.
   base::HistogramBase* open_histogram_ = nullptr;
   base::HistogramBase* db_restore_histogram_ = nullptr;
   base::HistogramBase* value_restore_histogram_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(LazyLevelDb);
 };
 
 #endif  // EXTENSIONS_BROWSER_VALUE_STORE_LAZY_LEVELDB_H_