Decouple X11WindowEventFilter into WindowEventFilter parent class.

ui/views/BUILD.gn

 # Copyright 2014 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/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 import("//ui/base/ui_features.gni")
 import("//ui/ozone/ozone.gni")
 import("//ui/vector_icons/vector_icons.gni")
@@ skipping 640 matching lines @@
           "widget/desktop_aura/desktop_drag_drop_client_win.h",
           "widget/desktop_aura/desktop_screen_win.cc",
           "widget/desktop_aura/desktop_screen_win.h",
           "widget/desktop_aura/desktop_window_tree_host_win.cc",
           "widget/desktop_aura/desktop_window_tree_host_win.h",
         ]
       } else if (use_ozone) {
         sources += [ "widget/desktop_aura/desktop_window_tree_host_ozone.cc" ]
       }
       if (is_linux) {
-        sources += [ "style/platform_style_linux.cc" ]
+        sources += [
+          "style/platform_style_linux.cc",
+          "widget/desktop_aura/window_event_filter.cc",
+          "widget/desktop_aura/window_event_filter.h",
+        ]
       }
     }
   }
 
   if (is_mac) {
     deps += [
       "//ui/accelerated_widget_mac",
       "//ui/events:dom_keycode_converter",
     ]
     libs = [
@@ skipping 438 matching lines @@
       "//ui/base:test_support",
       "//ui/compositor",
       "//ui/events:test_support",
       "//ui/gl:test_support",
       "//ui/resources",
       "//ui/resources:ui_test_pak",
       "//ui/strings",
     ]
   }
 }