Move optional.h to webrtc/api/

webrtc/modules/audio_processing/BUILD.gn

Index: webrtc/modules/audio_processing/BUILD.gn
diff --git a/webrtc/modules/audio_processing/BUILD.gn b/webrtc/modules/audio_processing/BUILD.gn
index e0719b4315b2ef720089f3f2eb9f913d8b188d6a..4887250d884ccd621d10f2cccd8025cd58f413af 100644
--- a/webrtc/modules/audio_processing/BUILD.gn
+++ b/webrtc/modules/audio_processing/BUILD.gn
@@ -238,6 +238,7 @@ rtc_static_library("audio_processing") {
     "..:module_api",
     "../..:webrtc_common",
     "../../api:array_view",
+    "../../api:optional",
     "../../audio/utility:audio_frame_operations",
     "../../rtc_base:gtest_prod",
     "../../rtc_base:protobuf_utils",
@@ -553,6 +554,7 @@ if (rtc_include_tests) {
       "..:module_api",
       "../..:webrtc_common",
       "../../api:array_view",
+      "../../api:optional",
       "../../common_audio:common_audio",
       "../../rtc_base:gtest_prod",
       "../../rtc_base:protobuf_utils",
@@ -747,6 +749,7 @@ if (rtc_include_tests) {
         ":audioproc_debug_proto",
         ":audioproc_protobuf_utils",
         ":audioproc_test_utils",
+        "../../api:optional",
         "../../common_audio:common_audio",
         "../../rtc_base:protobuf_utils",
         "../../rtc_base:rtc_base_approved",
@@ -778,6 +781,7 @@ if (rtc_include_tests) {
       ":audio_processing",
       "..:module_api",
       "../../api:array_view",
+      "../../api:optional",
       "../../common_audio",
       "../../rtc_base:rtc_base_approved",
       "../../system_wrappers:system_wrappers",