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

tools/gn/parse_tree.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_PARSE_TREE_H_
 #define TOOLS_GN_PARSE_TREE_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 313 matching lines @@
   scoped_ptr<BlockNode> block_;  // May be null.
 
   DISALLOW_COPY_AND_ASSIGN(FunctionCallNode);
 };
 
 // IdentifierNode --------------------------------------------------------------
 
 class IdentifierNode : public ParseNode {
  public:
   IdentifierNode();
-  IdentifierNode(const Token& token);
+  explicit IdentifierNode(const Token& token);
   ~IdentifierNode() override;
 
   const IdentifierNode* AsIdentifier() const override;
   Value Execute(Scope* scope, Err* err) const override;
   LocationRange GetRange() const override;
   Err MakeErrorDescribing(
       const std::string& msg,
       const std::string& help = std::string()) const override;
   void Print(std::ostream& out, int indent) const override;
 
@@ skipping 61 matching lines @@
   std::vector<const ParseNode*> contents_;
 
   DISALLOW_COPY_AND_ASSIGN(ListNode);
 };
 
 // LiteralNode -----------------------------------------------------------------
 
 class LiteralNode : public ParseNode {
  public:
   LiteralNode();
-  LiteralNode(const Token& token);
+  explicit LiteralNode(const Token& token);
   ~LiteralNode() override;
 
   const LiteralNode* AsLiteral() const override;
   Value Execute(Scope* scope, Err* err) const override;
   LocationRange GetRange() const override;
   Err MakeErrorDescribing(
       const std::string& msg,
       const std::string& help = std::string()) const override;
   void Print(std::ostream& out, int indent) const override;
 
@@ skipping 68 matching lines @@
 };
 
 // EndNode ---------------------------------------------------------------------
 
 // This node type is used as the end_ object for lists and blocks (rather than
 // just the end ']', '}', or ')' token). This is so that during formatting
 // traversal there is a node that appears at the end of the block to which
 // comments can be attached.
 class EndNode : public ParseNode {
  public:
-  EndNode(const Token& token);
+  explicit EndNode(const Token& token);
   ~EndNode() override;
 
   const EndNode* AsEnd() const override;
   Value Execute(Scope* scope, Err* err) const override;
   LocationRange GetRange() const override;
   Err MakeErrorDescribing(
       const std::string& msg,
       const std::string& help = std::string()) const override;
   void Print(std::ostream& out, int indent) const override;
 
   const Token& value() const { return value_; }
   void set_value(const Token& t) { value_ = t; }
 
  private:
   Token value_;
 
   DISALLOW_COPY_AND_ASSIGN(EndNode);
 };
 
 #endif  // TOOLS_GN_PARSE_TREE_H_