Move optional.h to webrtc/api/

webrtc/api/video_codecs/BUILD.gn

 # Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../../webrtc.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
 
 rtc_source_set("video_codecs_api") {
   sources = [
     "video_decoder.h",
     "video_encoder.cc",
     "video_encoder.h",
   ]
 
   deps = [
+    "..:optional",
     "..:video_frame_api",
     "../..:webrtc_common",
     "../../common_video",
     "../../rtc_base:rtc_base_approved",
   ]
 }