Update from https://crrev.com/315085

base/json/json_string_value_serializer.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.
 
 #ifndef BASE_JSON_JSON_STRING_VALUE_SERIALIZER_H_
 #define BASE_JSON_JSON_STRING_VALUE_SERIALIZER_H_
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
+#include "base/strings/string_piece.h"
 #include "base/values.h"
 
 class BASE_EXPORT JSONStringValueSerializer : public base::ValueSerializer {
  public:
-  // json_string is the string that will be source of the deserialization
+  // |json_string| is the string that will be source of the deserialization
   // or the destination of the serialization.  The caller of the constructor
-  // retains ownership of the string.
-  explicit JSONStringValueSerializer(std::string* json_string)
-      : json_string_(json_string),
-        initialized_with_const_string_(false),
-        pretty_print_(false),
-        allow_trailing_comma_(false) {
-  }
+  // retains ownership of the string. |json_string| must not be null.
+  explicit JSONStringValueSerializer(std::string* json_string);
 
-  // This version allows initialization with a const string reference for
-  // deserialization only.
-  explicit JSONStringValueSerializer(const std::string& json_string)
-      : json_string_(&const_cast<std::string&>(json_string)),
-        initialized_with_const_string_(true),
-        pretty_print_(false),
-        allow_trailing_comma_(false) {
-  }
+  // This version allows initialization with a StringPiece for deserialization
+  // only. Retains a reference to the contents of |json_string|, so the data
+  // must outlive the JSONStringValueSerializer.
+  explicit JSONStringValueSerializer(const base::StringPiece& json_string);
 
   ~JSONStringValueSerializer() override;
 
   // Attempt to serialize the data structure represented by Value into
   // JSON.  If the return value is true, the result will have been written
   // into the string passed into the constructor.
   bool Serialize(const base::Value& root) override;
 
   // Equivalent to Serialize(root) except binary values are omitted from the
   // output.
   bool SerializeAndOmitBinaryValues(const base::Value& root);
 
   // Attempt to deserialize the data structure encoded in the string passed
   // in to the constructor into a structure of Value objects.  If the return
-  // value is NULL, and if |error_code| is non-null, |error_code| will
+  // value is null, and if |error_code| is non-null, |error_code| will
   // contain an integer error code (a JsonParseError in this case).
   // If |error_message| is non-null, it will be filled in with a formatted
   // error message including the location of the error if appropriate.
   // The caller takes ownership of the returned value.
   base::Value* Deserialize(int* error_code,
                            std::string* error_message) override;
 
   void set_pretty_print(bool new_value) { pretty_print_ = new_value; }
   bool pretty_print() { return pretty_print_; }
 
   void set_allow_trailing_comma(bool new_value) {
     allow_trailing_comma_ = new_value;
   }
 
  private:
   bool SerializeInternal(const base::Value& root, bool omit_binary_values);
 
+  // String for writing. Owned by the caller of the constructor. Will be null if
+  // the serializer was initialized with a const string.
   std::string* json_string_;
-  bool initialized_with_const_string_;
+  // String for reading. Data is owned by the caller of the constructor. If
+  // |json_string_| is non-null, this is a view onto |json_string_|.
+  base::StringPiece json_string_readonly_;
   bool pretty_print_;  // If true, serialization will span multiple lines.
   // If true, deserialization will allow trailing commas.
   bool allow_trailing_comma_;
 
   DISALLOW_COPY_AND_ASSIGN(JSONStringValueSerializer);
 };
 
 #endif  // BASE_JSON_JSON_STRING_VALUE_SERIALIZER_H_