Add a test for the HexDecode method.

BUILD.gn

 # Copyright 2014 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.
 
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_config") {
   cflags = []
   include_dirs = [ "third_party/freetype/include" ]
@@ skipping 697 matching lines @@
     "fpdfsdk/src/formfiller/FFL_PushButton.cpp",
     "fpdfsdk/src/formfiller/FFL_RadioButton.cpp",
     "fpdfsdk/src/formfiller/FFL_TextField.cpp",
   ]
   configs += [ ":pdfium_config" ]
 }
 
 test("pdfium_unittests") {
   sources = [
     "core/src/fpdfapi/fpdf_font/fpdf_font_unittest.cpp",
+    "core/src/fpdfapi/fpdf_parser/fpdf_parser_decode_unittest.cpp",
     "core/src/fxcodec/codec/fx_codec_jpx_unittest.cpp",
     "core/src/fxcrt/fx_basic_bstring_unittest.cpp",
     "core/src/fxcrt/fx_basic_memmgr_unittest.cpp",
     "core/src/fxcrt/fx_basic_wstring_unittest.cpp",
     "core/src/fxcrt/fx_bidi_unittest.cpp",
     "core/src/fxcrt/fx_system_unittest.cpp",
     "third_party/base/nonstd_unique_ptr_unittest.cpp",
   ]
   deps = [
     "//testing/gtest",
@@ skipping 39 matching lines @@
       "//v8",
       "//v8:v8_libplatform",
     ]
     include_dirs += [
       "//v8",
       "//v8/include",
     ]
   }
   configs += [ ":pdfium_config" ]
 }