Unify the three CFWL_EventType::SelectChange classes.

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/core/cfwl_evtmouse.cpp",
       "xfa/fwl/core/cfwl_evtmouse.h",
       "xfa/fwl/core/cfwl_evtmousewheel.cpp",
       "xfa/fwl/core/cfwl_evtmousewheel.h",
       "xfa/fwl/core/cfwl_evtpostdropdown.cpp",
       "xfa/fwl/core/cfwl_evtpostdropdown.h",
       "xfa/fwl/core/cfwl_evtpredropdown.cpp",
       "xfa/fwl/core/cfwl_evtpredropdown.h",
       "xfa/fwl/core/cfwl_evtscroll.cpp",
       "xfa/fwl/core/cfwl_evtscroll.h",
+      "xfa/fwl/core/cfwl_evtselectchanged.cpp",
+      "xfa/fwl/core/cfwl_evtselectchanged.h",
       "xfa/fwl/core/cfwl_evtsetfocus.cpp",
       "xfa/fwl/core/cfwl_evtsetfocus.h",
       "xfa/fwl/core/cfwl_evtsizechanged.cpp",
       "xfa/fwl/core/cfwl_evtsizechanged.h",
       "xfa/fwl/core/cfwl_evttextchanged.cpp",
       "xfa/fwl/core/cfwl_evttextchanged.h",
       "xfa/fwl/core/cfwl_evttextfull.cpp",
       "xfa/fwl/core/cfwl_evttextfull.h",
       "xfa/fwl/core/cfwl_evtvalidate.cpp",
       "xfa/fwl/core/cfwl_evtvalidate.h",
@@ skipping 603 matching lines @@
       "//samples",
     ]
   }
   group("fuzzers") {
     testonly = true
     deps = [
       "//testing/libfuzzer",
     ]
   }
 }