Move the message definitions to their own files.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 831a5a07012e52f57f8a21eaa346d3cbb851093c..96967c3a164259a1582c9ca85f577e86c8913653 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1179,7 +1179,18 @@ if (pdf_enable_xfa) {
       "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",