Add GN build file for ui/aura target.

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") {
-  deps = [
-    "//base"
-  ]
-
-  defines = [
-    "EVENTS_IMPLEMENTATION",
-  ]
-
   sources = [
     "platform_event_dispatcher.h",
     "platform_event_observer.h",
     "platform_event_source.cc",
     "platform_event_source.h",
     "platform_event_source_stub.cc",
     "platform_event_types.h",
     "scoped_event_dispatcher.cc",
     "scoped_event_dispatcher.h",
   ]
 
+  defines = [
+    "EVENTS_IMPLEMENTATION",
+  ]
+
+  deps = [
+    "//base"
+  ]
+
   if (use_x11) {
-    sources -= [ "platform_event_source_stub.cc" ]
+    sources -= [
+      "platform_event_source_stub.cc",
+    ]
   }
 }