Move the message definitions to their own files.

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 1165 matching lines @@
       "xfa/fwl/core/cfwl_combobox.h",
       "xfa/fwl/core/cfwl_datetimepicker.cpp",
       "xfa/fwl/core/cfwl_datetimepicker.h",
       "xfa/fwl/core/cfwl_edit.cpp",
       "xfa/fwl/core/cfwl_edit.h",
       "xfa/fwl/core/cfwl_event.h",
       "xfa/fwl/core/cfwl_listbox.cpp",
       "xfa/fwl/core/cfwl_listbox.h",
       "xfa/fwl/core/cfwl_listitem.cpp",
       "xfa/fwl/core/cfwl_listitem.h",
+      "xfa/fwl/core/cfwl_message.cpp",
       "xfa/fwl/core/cfwl_message.h",
+      "xfa/fwl/core/cfwl_msgkey.cpp",
+      "xfa/fwl/core/cfwl_msgkey.h",
+      "xfa/fwl/core/cfwl_msgkillfocus.cpp",
+      "xfa/fwl/core/cfwl_msgkillfocus.h",
+      "xfa/fwl/core/cfwl_msgmouse.cpp",
+      "xfa/fwl/core/cfwl_msgmouse.h",
+      "xfa/fwl/core/cfwl_msgmousewheel.cpp",
+      "xfa/fwl/core/cfwl_msgmousewheel.h",
+      "xfa/fwl/core/cfwl_msgsetfocus.cpp",
+      "xfa/fwl/core/cfwl_msgsetfocus.h",
       "xfa/fwl/core/cfwl_picturebox.cpp",
       "xfa/fwl/core/cfwl_picturebox.h",
       "xfa/fwl/core/cfwl_pushbutton.cpp",
       "xfa/fwl/core/cfwl_pushbutton.h",
       "xfa/fwl/core/cfwl_themebackground.h",
       "xfa/fwl/core/cfwl_themepart.h",
       "xfa/fwl/core/cfwl_themetext.h",
       "xfa/fwl/core/cfwl_widget.cpp",
       "xfa/fwl/core/cfwl_widget.h",
       "xfa/fwl/core/cfwl_widgetmgr.cpp",
@@ skipping 577 matching lines @@
       "//samples",
     ]
   }
   group("fuzzers") {
     testonly = true
     deps = [
       "//testing/libfuzzer",
     ]
   }
 }