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/generic_encoder.cc b/webrtc/modules/video_coding/generic_encoder.cc
index adc4365..339e1b5 100644
--- a/webrtc/modules/video_coding/generic_encoder.cc
+++ b/webrtc/modules/video_coding/generic_encoder.cc
@@ -12,13 +12,13 @@
 
 #include <vector>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/api/video/i420_buffer.h"
 #include "webrtc/modules/pacing/alr_detector.h"
 #include "webrtc/modules/video_coding/encoded_frame.h"
 #include "webrtc/modules/video_coding/media_optimization.h"
 #include "webrtc/rtc_base/checks.h"
 #include "webrtc/rtc_base/logging.h"
-#include "webrtc/rtc_base/optional.h"
 #include "webrtc/rtc_base/timeutils.h"
 #include "webrtc/rtc_base/trace_event.h"
 #include "webrtc/system_wrappers/include/field_trial.h"