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/audio_processing/rms_level.h

@@ -12,7 +12,7 @@
 #define WEBRTC_MODULES_AUDIO_PROCESSING_RMS_LEVEL_H_
 
 #include "webrtc/api/array_view.h"
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {