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

tools/gn/substitution_pattern.h

 // Copyright 2014 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_SUBSTITUTION_PATTERN_H_
 #define TOOLS_GN_SUBSTITUTION_PATTERN_H_
 
 #include <string>
 #include <vector>
 
 #include "tools/gn/substitution_type.h"
 
 class BuildSettings;
 class Err;
 class ParseNode;
 class Value;
 
 // Represents a string with {{substitution_patterns}} in them.
 class SubstitutionPattern {
  public:
   struct Subrange {
     Subrange();
-    Subrange(SubstitutionType t, const std::string& l = std::string());
+    explicit Subrange(SubstitutionType t, const std::string& l = std::string());
     ~Subrange();
 
     inline bool operator==(const Subrange& other) const {
       return type == other.type && literal == other.literal;
     }
 
     SubstitutionType type;
 
     // When type_ == LITERAL, this specifies the literal.
     std::string literal;
@@ skipping 34 matching lines @@
   bool empty() const { return ranges_.empty(); }
 
  private:
   std::vector<Subrange> ranges_;
   const ParseNode* origin_;
 
   std::vector<SubstitutionType> required_types_;
 };
 
 #endif  // TOOLS_GN_SUBSTITUTION_PATTERN_H_