Add a fuzzer for gn's parser.

tools/gn/parser_fuzzer.cc

+// Copyright 2016 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.
+
+#include <stdint.h>
+
+#include "tools/gn/input_file.h"
+#include "tools/gn/parser.h"
+#include "tools/gn/source_file.h"
+#include "tools/gn/tokenizer.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data, size_t size) {
+  SourceFile source;
+  InputFile input(source);
+  input.SetContents(std::string(reinterpret_cast<const char*>(data), size));
+
+  Err err;
+  std::vector<Token> tokens = Tokenizer::Tokenize(&input, &err);
+
+  if (!err.has_error())
+    Parser::Parse(tokens, &err);
+
+  return 0;
+}