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/sdk/BUILD.gn b/webrtc/sdk/BUILD.gn
index ff97082..086ce49 100644
--- a/webrtc/sdk/BUILD.gn
+++ b/webrtc/sdk/BUILD.gn
@@ -183,6 +183,7 @@
         ":common_objc",
         ":videotracksource_objc",
         "../api:libjingle_peerconnection_api",
+        "../api:optional",
         "../common_video",
         "../media:rtc_media_base",
         "../rtc_base:rtc_base",
diff --git a/webrtc/sdk/objc/Framework/Classes/Video/RTCDefaultShader.mm b/webrtc/sdk/objc/Framework/Classes/Video/RTCDefaultShader.mm
index 900f284..14301ba 100644
--- a/webrtc/sdk/objc/Framework/Classes/Video/RTCDefaultShader.mm
+++ b/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;