Decouple X11WindowEventFilter into WindowEventFilter parent class.

ui/views/BUILD.gn

Index: ui/views/BUILD.gn
diff --git a/ui/views/BUILD.gn b/ui/views/BUILD.gn
index 144bd6ba7cab42bc67170c0ff74d5e2dd107789f..be51a992e45410f8eea44b9a50f9203b8cef69fa 100644
--- a/ui/views/BUILD.gn
+++ b/ui/views/BUILD.gn
@@ -658,7 +658,11 @@ component("views") {
         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",
+        ]
       }
     }
   }