Add fuzzer for cmap parsing

testing/libfuzzer/pdf_cmap_fuzzer.cc

 // Copyright 2016 The PDFium 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 "testing/libfuzzer/xfa_codec_fuzzer.h"
+#include <cstdint>
+
+#include "core/fpdfapi/fpdf_font/font_int.h"
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
-  return XFACodecFuzzer::Fuzz(data, size, FXCODEC_IMAGE_JPG);
+  CPDF_CMap cmap;
+  cmap.LoadEmbedded(data, size);
+  return 0;
 }