Move optional.h to webrtc/api/

webrtc/modules/pacing/BUILD.gn

 # Copyright (c) 2014 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")
 
@@ skipping 13 matching lines @@
   ]
 
   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" ]
   }
 
   deps = [
     "..:module_api",
     "../../:webrtc_common",
+    "../../api:optional",
     "../../logging:rtc_event_log_api",
     "../../rtc_base:rtc_base_approved",
     "../../system_wrappers",
     "../remote_bitrate_estimator",
     "../rtp_rtcp",
     "../utility",
   ]
 }
 
 if (rtc_include_tests) {
@@ skipping 37 matching lines @@
     sources = [
       "mock/mock_paced_sender.h",
     ]
     deps = [
       ":pacing",
       "../../system_wrappers",
       "../../test:test_support",
     ]
   }
 }