[Ozone] Update GYP and GN to include a DRM platform

ui/ozone/ozone.gni

 # 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.
 
 declare_args() {
   # Select platforms automatically. Turn this off for manual control.
   ozone_auto_platforms = true
 }
 
 declare_args() {
   # The platform that will be active by default.
   ozone_platform = ""
 
   # Enable individual platforms.
   ozone_platform_caca = false
   ozone_platform_dri = false
+  ozone_platform_drm = false
   ozone_platform_egltest = false
   ozone_platform_gbm = false
   ozone_platform_test = false
 
   if (ozone_auto_platforms) {
     # Use test as the default platform.
     ozone_platform = "test"
 
     # Build all platforms whose deps are in install-build-deps.sh.
     # Only these platforms will be compile tested by buildbots.
     ozone_platform_dri = true
+    ozone_platform_drm = true
     ozone_platform_test = true
     ozone_platform_egltest = true
   }
 }