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/utility/quality_scaler.h

@@ -13,10 +13,10 @@
 
 #include <utility>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/video_coding/utility/moving_average.h"
-#include "webrtc/rtc_base/optional.h"
 #include "webrtc/rtc_base/sequenced_task_checker.h"
 
 namespace webrtc {