Move optional.h to webrtc/api/

We use Optional in our public API, so its header should be in
webrtc/api/.

BUG=webrtc:8205

Review-Url: https://codereview.webrtc.org/3011943002
Cr-Commit-Position: refs/heads/master@{#19693}
diff --git a/webrtc/api/audio_codecs/opus/BUILD.gn b/webrtc/api/audio_codecs/opus/BUILD.gn
index 658d151..faa5956 100644
--- a/webrtc/api/audio_codecs/opus/BUILD.gn
+++ b/webrtc/api/audio_codecs/opus/BUILD.gn
@@ -18,6 +18,7 @@
     "audio_encoder_opus_config.h",
   ]
   deps = [
+    "../..:optional",
     "../../../rtc_base:rtc_base_approved",
   ]
   defines = []
@@ -51,6 +52,7 @@
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:webrtc_opus",
     "../../../rtc_base:rtc_base_approved",