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

tools/gn/pattern.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_PATTERN_H_
 #define TOOLS_GN_PATTERN_H_
 
 #include <string>
 #include <vector>
 
 #include "tools/gn/value.h"
 
 class Pattern {
  public:
   struct Subrange {
     enum Type {
       LITERAL,  // Matches exactly the contents of the string.
       ANYTHING,  // * (zero or more chars).
       PATH_BOUNDARY  // '/' or beginning of string.
     };
 
-    Subrange(Type t, const std::string& l = std::string())
+    explicit Subrange(Type t, const std::string& l = std::string())
         : type(t),
           literal(l) {
     }
 
     // Returns the minimum number of chars that this subrange requires.
     size_t MinSize() const {
       switch (type) {
         case LITERAL:
           return literal.size();
         case ANYTHING:
           return 0;
         case PATH_BOUNDARY:
           return 0;  // Can match beginning or end of string, which is 0 len.
         default:
           return 0;
       }
     }
 
     Type type;
 
     // When type == LITERAL this is the text to match.
     std::string literal;
   };
 
-  Pattern(const std::string& s);
+  explicit Pattern(const std::string& s);
   ~Pattern();
 
   // Returns true if the current pattern matches the given string.
   bool MatchesString(const std::string& s) const;
 
  private:
   // allow_implicit_path_boundary determines if a path boundary should accept
   // matches at the beginning or end of the string.
   bool RecursiveMatch(const std::string& s,
                       size_t begin_char,
@@ skipping 21 matching lines @@
   void SetFromValue(const Value& v, Err* err);
 
   bool MatchesString(const std::string& s) const;
   bool MatchesValue(const Value& v) const;
 
  private:
   std::vector<Pattern> patterns_;
 };
 
 #endif  // TOOLS_GN_PATTERN_H_