Move optional.h to webrtc/api/

webrtc/sdk/BUILD.gn

 # Copyright 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")
 if (is_ios) {
@@ skipping 165 matching lines @@
           "CoreMedia.framework",
           "CoreVideo.framework",
           "OpenGL.framework",
         ]
       }
 
       deps = [
         ":common_objc",
         ":videotracksource_objc",
         "../api:libjingle_peerconnection_api",
+        "../api:optional",
         "../common_video",
         "../media:rtc_media_base",
         "../rtc_base:rtc_base",
       ]
 
       configs += [ "..:common_objc" ]
       if (!build_with_chromium && is_clang) {
         # Suppress warnings from the Chromium Clang plugin
         # (bugs.webrtc.org/163).
         suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
@@ skipping 572 matching lines @@
         public_deps = [
           "$rtc_libyuv_dir",
         ]
       } else {
         # Need to add a directory normally exported by libyuv.
         include_dirs = [ "$rtc_libyuv_dir/include" ]
       }
     }
   }
 }