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/ortc/rtptransportinterface.h b/webrtc/api/ortc/rtptransportinterface.h
index 165daad..1fdc12c 100644
--- a/webrtc/api/ortc/rtptransportinterface.h
+++ b/webrtc/api/ortc/rtptransportinterface.h
@@ -13,10 +13,10 @@
 
 #include <string>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/api/ortc/packettransportinterface.h"
 #include "webrtc/api/rtcerror.h"
 #include "webrtc/common_types.h"
-#include "webrtc/rtc_base/optional.h"
 
 namespace webrtc {