Move optional.h to webrtc/api/

webrtc/api/audio_codecs/isac/BUILD.gn

Index: webrtc/api/audio_codecs/isac/BUILD.gn
diff --git a/webrtc/api/audio_codecs/isac/BUILD.gn b/webrtc/api/audio_codecs/isac/BUILD.gn
index 3340e28a06a8830400b91e9731cdbf14a93e8f93..b22eb9552585f5f1e9c12c51bdb3e33ef337631a 100644
--- a/webrtc/api/audio_codecs/isac/BUILD.gn
+++ b/webrtc/api/audio_codecs/isac/BUILD.gn
@@ -19,6 +19,7 @@ rtc_static_library("audio_encoder_isac_fix") {
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:isac_fix",
     "../../../rtc_base:rtc_base_approved",
@@ -32,6 +33,7 @@ rtc_static_library("audio_decoder_isac_fix") {
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:isac_fix",
     "../../../rtc_base:rtc_base_approved",
@@ -45,6 +47,7 @@ rtc_static_library("audio_encoder_isac_float") {
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:isac",
     "../../../rtc_base:rtc_base_approved",
@@ -58,6 +61,7 @@ rtc_static_library("audio_decoder_isac_float") {
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:isac",
     "../../../rtc_base:rtc_base_approved",