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/sdk/BUILD.gn b/webrtc/sdk/BUILD.gn
index ff97082..086ce49 100644
--- a/webrtc/sdk/BUILD.gn
+++ b/webrtc/sdk/BUILD.gn
@@ -183,6 +183,7 @@
         ":common_objc",
         ":videotracksource_objc",
         "../api:libjingle_peerconnection_api",
+        "../api:optional",
         "../common_video",
         "../media:rtc_media_base",
         "../rtc_base:rtc_base",