gn: Swap the order of two declarations.

Backports
https://chromium-review.googlesource.com/c/chromium/src/+/1098515 which
was made to src.git/tools/gn after forking to gn.googlesource.com.

Change-Id: Ie7488b31d824ab1caa631c955373842eb2c16b3a
Reviewed-on: https://gn-review.googlesource.com/1840
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>

tools/gn/parser.h

diff --git a/tools/gn/parser.h b/tools/gn/parser.h
index 69dc8cf..c1930b9 100644
--- a/tools/gn/parser.h
+++ b/tools/gn/parser.h
@@ -16,18 +16,9 @@
 #include "tools/gn/err.h"
 #include "tools/gn/parse_tree.h"
 
-class Parser;
-typedef std::unique_ptr<ParseNode> (Parser::*PrefixFunc)(const Token& token);
-typedef std::unique_ptr<ParseNode> (
-    Parser::*InfixFunc)(std::unique_ptr<ParseNode> left, const Token& token);
-
 extern const char kGrammar_Help[];
 
-struct ParserHelper {
-  PrefixFunc prefix;
-  InfixFunc infix;
-  int precedence;
-};
+struct ParserHelper;
 
 // Parses a series of tokens. The resulting AST will refer to the tokens passed
 // to the input, so the tokens an the file data they refer to must outlive your
@@ -146,4 +137,14 @@
   DISALLOW_COPY_AND_ASSIGN(Parser);
 };
 
+typedef std::unique_ptr<ParseNode> (Parser::*PrefixFunc)(const Token& token);
+typedef std::unique_ptr<ParseNode> (
+    Parser::*InfixFunc)(std::unique_ptr<ParseNode> left, const Token& token);
+
+struct ParserHelper {
+  PrefixFunc prefix;
+  InfixFunc infix;
+  int precedence;
+};
+
 #endif  // TOOLS_GN_PARSER_H_