Split the event library into a cross-platform and native part. Part 1.

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_builder.cc",
+    "platform_event_builder.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",
+
+    # TODO(erg): Figure out what to do with this dependency later. Maybe get to
+    # the point where we can remove it before we commit this.
+    "//ui/events",
   ]
 
   if (use_x11) {
     sources -= [ "platform_event_source_stub.cc" ]
   }
 }