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/rtp_headers.h

@@ -16,8 +16,8 @@
 #include <string>
 #include <vector>
 
+#include "absl/types/optional.h"
 #include "api/array_view.h"
-#include "api/optional.h"
 #include "api/video/video_content_type.h"
 #include "api/video/video_rotation.h"
 #include "api/video/video_timing.h"
@@ -102,7 +102,7 @@ struct RTPHeaderExtension {
   bool hasVideoRotation;
   VideoRotation videoRotation;
 
-  // TODO(ilnik): Refactor this and one above to be rtc::Optional() and remove
+  // TODO(ilnik): Refactor this and one above to be absl::optional() and remove
   // a corresponding bool flag.
   bool hasVideoContentType;
   VideoContentType videoContentType;