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/sdk/objc/Framework/Classes/Video/RTCDefaultShader.mm

@@ -20,7 +20,7 @@
 #import "RTCShader.h"
 #import "WebRTC/RTCLogging.h"
 
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 
 static const int kYTextureUnit = 0;
 static const int kUTextureUnit = 1;