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") {
+  output_name = "events_platform"
+
+  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",
+    ]
+  }
+}