Move optional.h to webrtc/api/

webrtc/modules/BUILD.gn

 # Copyright (c) 2016 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")
 import("audio_coding/audio_coding.gni")
@@ skipping 18 matching lines @@
   ]
 }
 
 rtc_source_set("module_api") {
   sources = [
     "include/module.h",
     "include/module_common_types.h",
   ]
   deps = [
     "..:webrtc_common",
+    "../api:optional",
     "../api:video_frame_api",
     "../rtc_base:rtc_base_approved",
   ]
 }
 
 if (rtc_include_tests) {
   modules_tests_resources = [
     "../../resources/audio_coding/testfile32kHz.pcm",
     "../../resources/audio_coding/teststereo32kHz.pcm",
     "../../resources/foreman_cif.yuv",
@@ skipping 202 matching lines @@
       shard_timeout = 900
     }
     if (is_ios) {
       info_plist = "../test/ios/Info.plist"
       deps += [ ":modules_unittests_bundle_data" ]
       configs += [ "..:common_objc" ]
       ldflags = [ "-ObjC" ]
     }
   }
 }