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

tools/gn/parse_tree.h

Index: tools/gn/parse_tree.h
diff --git a/tools/gn/parse_tree.h b/tools/gn/parse_tree.h
index e9817a8e9fc7b26afc8dea857851389d9cb7dce3..0611ce1f95979ad5a9a5f24a6ea0a27dc40ba525 100644
--- a/tools/gn/parse_tree.h
+++ b/tools/gn/parse_tree.h
@@ -331,7 +331,7 @@ class FunctionCallNode : public ParseNode {
 class IdentifierNode : public ParseNode {
  public:
   IdentifierNode();
-  IdentifierNode(const Token& token);
+  explicit IdentifierNode(const Token& token);
   ~IdentifierNode() override;
 
   const IdentifierNode* AsIdentifier() const override;
@@ -413,7 +413,7 @@ class ListNode : public ParseNode {
 class LiteralNode : public ParseNode {
  public:
   LiteralNode();
-  LiteralNode(const Token& token);
+  explicit LiteralNode(const Token& token);
   ~LiteralNode() override;
 
   const LiteralNode* AsLiteral() const override;
@@ -502,7 +502,7 @@ class BlockCommentNode : public ParseNode {
 // 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;