Fix subtle issues in opj_skip_from_memory and add unit tests.

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")
 
 pdf_use_skia = false
 
 declare_args() {
   # On Android there's no system FreeType. On Windows and Mac, only a few
@@ skipping 828 matching lines @@
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }
 
 test("pdfium_unittests") {
   sources = [
     "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_wstring_unittest.cpp",
     "testing/fx_string_testhelpers.cpp",
     "testing/fx_string_testhelpers.h",
   ]
   deps = [
     "//testing/gtest",
     "//testing/gtest:gtest_main",
     ":pdfium",
   ]
@@ skipping 23 matching lines @@
     "//testing/gtest",
     ":pdfium",
   ]
   include_dirs = [ "." ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }