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/mediastreaminterface.h

@@ -22,7 +22,7 @@
 #include <string>
 #include <vector>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 #include "api/video/video_frame.h"
 // TODO(zhihuang): Remove unrelated headers once downstream applications stop
 // relying on them; they were previously transitively included by
@@ -132,7 +132,7 @@ class VideoTrackSourceInterface : public MediaSourceInterface,
   // depending on video codec.
   // TODO(perkj): Remove this once denoising is done by the source, and not by
   // the encoder.
-  virtual rtc::Optional<bool> needs_denoising() const = 0;
+  virtual absl::optional<bool> needs_denoising() const = 0;
 
   // Returns false if no stats are available, e.g, for a remote source, or a
   // source which has not seen its first frame yet.