Fix 'gn check' errors in //ui/* and add to default checked set

ui/events/platform/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/ui.gni")
 
 component("platform") {
   sources = [
     # Allow this target to include events_export.h without depending on the
     # events target (which would be circular).
@@ skipping 34 matching lines @@
       "x11/keysym_to_unicode.cc",
       "x11/keysym_to_unicode.h",
       "x11/platform_event_utils_x11.cc",
       "x11/touch_factory_x11.cc",
       "x11/touch_factory_x11.h",
       "x11/x11_event_source.cc",
       "x11/x11_event_source.h",
       "x11/x11_event_source_libevent.cc",
     ]
 
-    deps += [ "//ui/gfx/x" ]
+    deps += [
+      "//ui/events:dom4_keycode_converter",
+      "//ui/gfx/x",
+    ]
   } else {
     sources += [
       "platform_event_source_stub.cc",
       "platform_event_stub.cc",
     ]
   }
 }