Remove fgas_base.h and IWYU

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_config") {
   cflags = []
@@ skipping 903 matching lines @@
       "xfa/src/fgas/src/crt/fx_algorithm.cpp",
       "xfa/src/fgas/src/crt/fx_codepage.cpp",
       "xfa/src/fgas/src/crt/fx_encode.cpp",
       "xfa/src/fgas/src/crt/fx_memory.cpp",
       "xfa/src/fgas/src/crt/fx_memory.h",
       "xfa/src/fgas/src/crt/fx_stream.cpp",
       "xfa/src/fgas/src/crt/fx_stream.h",
       "xfa/src/fgas/src/crt/fx_system.cpp",
       "xfa/src/fgas/src/crt/fx_utils.cpp",
       "xfa/src/fgas/src/crt/fx_utils.h",
-      "xfa/src/fgas/src/fgas_base.h",
       "xfa/src/fgas/src/font/fx_fontutils.cpp",
       "xfa/src/fgas/src/font/fx_fontutils.h",
       "xfa/src/fgas/src/font/fx_gefont.cpp",
       "xfa/src/fgas/src/font/fx_gefont.h",
       "xfa/src/fgas/src/font/fx_stdfontmgr.cpp",
       "xfa/src/fgas/src/font/fx_stdfontmgr.h",
       "xfa/src/fgas/src/layout/fx_linebreak.cpp",
       "xfa/src/fgas/src/layout/fx_rtfbreak.cpp",
       "xfa/src/fgas/src/layout/fx_rtfbreak.h",
       "xfa/src/fgas/src/layout/fx_textbreak.cpp",
@@ skipping 570 matching lines @@
       "//v8:v8_libplatform",
     ]
     include_dirs += [
       "//v8",
       "//v8/include",
     ]
     configs += [ "//v8:external_startup_data" ]
   }
   configs += [ ":pdfium_config" ]
 }