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

tools/gn/test_with_scope.h

 // 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.
 
 #ifndef TOOLS_GN_TEST_WITH_SCOPE_H_
 #define TOOLS_GN_TEST_WITH_SCOPE_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 42 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(TestWithScope);
 };
 
 // Helper class to treat some string input as a file.
 //
 // Instantiate it with the contents you want, be sure to check for error, and
 // then you can execute the ParseNode or whatever.
 class TestParseInput {
  public:
-  TestParseInput(const std::string& input);
+  explicit TestParseInput(const std::string& input);
   ~TestParseInput();
 
   // Indicates whether and what error occurred during tokenizing and parsing.
   bool has_error() const { return parse_err_.has_error(); }
   const Err& parse_err() const { return parse_err_; }
 
   const InputFile& input_file() const { return input_file_; }
   const std::vector<Token>& tokens() const { return tokens_; }
   const ParseNode* parsed() const { return parsed_.get(); }
 
  private:
   InputFile input_file_;
 
   std::vector<Token> tokens_;
   scoped_ptr<ParseNode> parsed_;
 
   Err parse_err_;
 
   DISALLOW_COPY_AND_ASSIGN(TestParseInput);
 };
 
 #endif  // TOOLS_GN_TEST_WITH_SCOPE_H_