tools/gn: don't allow trailing junk or non-literals in "value" conversion

tools/gn/parser.h

Index: tools/gn/parser.h
diff --git a/tools/gn/parser.h b/tools/gn/parser.h
index ce24e0dffd8f54cdc1661abafdd3f4b6e4013bc2..3348e2d225505033af347634c9befbd73d8f416e 100644
--- a/tools/gn/parser.h
+++ b/tools/gn/parser.h
@@ -40,13 +40,17 @@ class Parser {
   static scoped_ptr<ParseNode> ParseExpression(const std::vector<Token>& tokens,
                                                Err* err);
 
-  scoped_ptr<ParseNode> ParseExpression();
+  // Alternative to parsing that assumes the input is a literal value.
+  static scoped_ptr<ParseNode> ParseValue(const std::vector<Token>& tokens,
+                                          Err* err);
 
  private:
   // Vector must be valid for lifetime of call.
   Parser(const std::vector<Token>& tokens, Err* err);
   ~Parser();
 
+  scoped_ptr<ParseNode> ParseExpression();
+
   // Parses an expression with the given precedence or higher.
   scoped_ptr<ParseNode> ParseExpression(int precedence);