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/mediaconstraintsinterface.h b/webrtc/api/mediaconstraintsinterface.h
index 72fdd97..eb89b3e 100644
--- a/webrtc/api/mediaconstraintsinterface.h
+++ b/webrtc/api/mediaconstraintsinterface.h
@@ -23,8 +23,8 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/api/peerconnectioninterface.h"
-#include "webrtc/rtc_base/optional.h"
 
 namespace webrtc {