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/audio/time_interval.h b/webrtc/audio/time_interval.h
index 069127d..4481843 100644
--- a/webrtc/audio/time_interval.h
+++ b/webrtc/audio/time_interval.h
@@ -13,7 +13,7 @@
#include <stdint.h>
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
namespace webrtc {