Merge IFWL_App and FWL_AppImp together

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 1186 matching lines @@
       "xfa/fwl/basewidget/ifwl_spinbutton.h",
       "xfa/fwl/basewidget/ifwl_tooltip.h",
       "xfa/fwl/core/cfwl_event.h",
       "xfa/fwl/core/cfwl_message.h",
       "xfa/fwl/core/cfwl_themebackground.h",
       "xfa/fwl/core/cfwl_themepart.h",
       "xfa/fwl/core/cfwl_themetext.h",
       "xfa/fwl/core/cfwl_widgetimpproperties.h",
       "xfa/fwl/core/cfwl_widgetmgr.cpp",
       "xfa/fwl/core/cfwl_widgetmgr.h",
-      "xfa/fwl/core/fwl_appimp.cpp",
-      "xfa/fwl/core/fwl_appimp.h",
       "xfa/fwl/core/fwl_error.h",
       "xfa/fwl/core/fwl_formimp.cpp",
       "xfa/fwl/core/fwl_formimp.h",
       "xfa/fwl/core/fwl_noteimp.cpp",
       "xfa/fwl/core/fwl_noteimp.h",
       "xfa/fwl/core/fwl_timerimp.cpp",
       "xfa/fwl/core/fwl_widgetdef.h",
       "xfa/fwl/core/fwl_widgethit.h",
       "xfa/fwl/core/fwl_widgetimp.cpp",
       "xfa/fwl/core/fwl_widgetimp.h",
       "xfa/fwl/core/ifwl_adaptertimermgr.h",
+      "xfa/fwl/core/ifwl_app.cpp",
       "xfa/fwl/core/ifwl_app.h",
       "xfa/fwl/core/ifwl_dataprovider.h",
       "xfa/fwl/core/ifwl_form.h",
       "xfa/fwl/core/ifwl_themeprovider.h",
       "xfa/fwl/core/ifwl_timer.h",
       "xfa/fwl/core/ifwl_widget.h",
       "xfa/fwl/core/ifwl_widgetdelegate.h",
       "xfa/fwl/lightwidget/cfwl_barcode.cpp",
       "xfa/fwl/lightwidget/cfwl_barcode.h",
       "xfa/fwl/lightwidget/cfwl_checkbox.cpp",
@@ skipping 527 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }