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/api/video_codecs/BUILD.gn

@@ -20,6 +20,7 @@ rtc_source_set("video_codecs_api") {
   ]
 
   deps = [
+    "..:optional",
     "..:video_frame_api",
     "../..:webrtc_common",
     "../../common_video",

webrtc/api/video_codecs/video_encoder.h

@@ -15,11 +15,11 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/api/video/video_frame.h"
 #include "webrtc/common_types.h"
 #include "webrtc/common_video/include/video_frame.h"
 #include "webrtc/rtc_base/checks.h"
-#include "webrtc/rtc_base/optional.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {