tools/gn: Fix errors found by Facebook's flint tool.

tools/gn/config_values_extractors.cc

 // Copyright (c) 2013 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.
 
 #include "tools/gn/config_values_extractors.h"
 
 #include "tools/gn/escape.h"
 
 namespace {
 
 class EscapedStringWriter {
  public:
-  EscapedStringWriter(const EscapeOptions& escape_options)
+  explicit EscapedStringWriter(const EscapeOptions& escape_options)
       : escape_options_(escape_options) {
   }
 
   void operator()(const std::string& s, std::ostream& out) const {
     out << " ";
     EscapeStringToStream(out, s, escape_options_);
   }
 
  private:
   const EscapeOptions& escape_options_;
 };
 
 }  // namespace
 
 void RecursiveTargetConfigStringsToStream(
     const Target* target,
     const std::vector<std::string>& (ConfigValues::* getter)() const,
     const EscapeOptions& escape_options,
     std::ostream& out) {
   RecursiveTargetConfigToStream(target, getter,
                                 EscapedStringWriter(escape_options), out);
 }