Add unit test for CXFA_FileRead.

BUILD.gn

 # 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.
 
 import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_common_config") {
   cflags = []
@@ skipping 1702 matching lines @@
     ":test_support",
     "//testing/gtest",
     "//testing/gtest:gtest_main",
   ]
   include_dirs = []
   if (pdf_enable_xfa) {
     sources += [
       "xfa/fde/css/fde_cssdatatable_unittest.cpp",
       "xfa/fde/xml/fde_xml_imp_unittest.cpp",
       "xfa/fxbarcode/pdf417/BC_PDF417HighLevelEncoder_unittest.cpp",
+      "xfa/fxfa/app/xfa_ffapp_unittest.cpp",
       "xfa/fxfa/app/xfa_textlayout_unittest.cpp",
       "xfa/fxfa/fm2js/xfa_simpleexpression_unittest.cpp",
       "xfa/fxfa/parser/xfa_utils_unittest.cpp",
     ]
   }
   if (pdf_use_skia || pdf_use_skia_paths) {
     sources += [ "core/fxge/skia/fx_skia_device_unittest.cpp" ]
     deps += [ "//skia" ]
   }
   if (pdf_enable_v8) {
@@ skipping 71 matching lines @@
       "//samples",
     ]
   }
   group("fuzzers") {
     testonly = true
     deps = [
       "//testing/libfuzzer",
     ]
   }
 }