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/rtc_base/string_to_number.h b/webrtc/rtc_base/string_to_number.h
index c61d1ed..93bd850 100644
--- a/webrtc/rtc_base/string_to_number.h
+++ b/webrtc/rtc_base/string_to_number.h
@@ -14,7 +14,7 @@
 #include <string>
 #include <limits>
 
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 
 namespace rtc {