Replace rtc::Optional with absl::optional in api

This is a no-op change because rtc::Optional is an alias to absl::optional

This CL generated by running script with parameter 'api'
Then undo changes to optional target itself and optional_unittests

find $@ -type f \( -name \*.h -o -name \*.cc \) \
-exec sed -i 's|rtc::Optional|absl::optional|g' {} \+ \
-exec sed -i 's|rtc::nullopt|absl::nullopt|g' {} \+ \
-exec sed -i 's|#include "api/optional.h"|#include "absl/types/optional.h"|' {} \+

find $@ -type f -name BUILD.gn \
-exec sed -r -i 's|"[\./api]*:optional"|"//third_party/abseil-cpp/absl/types:optional"|' {} \+;

git cl format

Bug: webrtc:9078
Change-Id: I44093da213369d6a502e33792c694f620f53b779
Reviewed-on: https://webrtc-review.googlesource.com/84621
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23707}

api/video_codecs/video_decoder.h

@@ -39,8 +39,8 @@ class DecodedImageCallback {
   // TODO(sakal): Remove other implementations when upstream projects have been
   // updated.
   virtual void Decoded(VideoFrame& decodedImage,
-                       rtc::Optional<int32_t> decode_time_ms,
-                       rtc::Optional<uint8_t> qp);
+                       absl::optional<int32_t> decode_time_ms,
+                       absl::optional<uint8_t> qp);
 
   virtual int32_t ReceivedDecodedReferenceFrame(const uint64_t pictureId);