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/modules/audio_processing/rms_level.h b/webrtc/modules/audio_processing/rms_level.h
index 4ce28a4..e0df754 100644
--- a/webrtc/modules/audio_processing/rms_level.h
+++ b/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 {