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/BUILD.gn b/webrtc/api/audio_codecs/BUILD.gn
index 29238e6..04a5238 100644
--- a/webrtc/api/audio_codecs/BUILD.gn
+++ b/webrtc/api/audio_codecs/BUILD.gn
@@ -27,6 +27,7 @@
   ]
   deps = [
     "..:array_view",
+    "..:optional",
     "../..:webrtc_common",
     "../../rtc_base:rtc_base_approved",
   ]