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}

webrtc/modules/video_coding/sequence_number_util.h

@@ -14,8 +14,8 @@
 #include <limits>
 #include <type_traits>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/rtc_base/mod_ops.h"
-#include "webrtc/rtc_base/optional.h"
 #include "webrtc/rtc_base/safe_compare.h"
 
 namespace webrtc {