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/video_coding/utility/moving_average.h b/webrtc/modules/video_coding/utility/moving_average.h
index bc31cd5..0f2c4cc 100644
--- a/webrtc/modules/video_coding/utility/moving_average.h
+++ b/webrtc/modules/video_coding/utility/moving_average.h
@@ -13,7 +13,7 @@
 
 #include <vector>
 
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 
 namespace webrtc {
 class MovingAverage {