Fix most of the "gn check" errors in ui/*

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).
+    "../events_export.h",
     "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"
+    "//base",
   ]
 
   if (use_x11) {
     sources -= [
       "platform_event_source_stub.cc",
     ]
   }
 }