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/rtptransceiverinterface.h

@@ -14,8 +14,8 @@
 #include <string>
 #include <vector>
 
+#include "absl/types/optional.h"
 #include "api/array_view.h"
-#include "api/optional.h"
 #include "api/rtpreceiverinterface.h"
 #include "api/rtpsenderinterface.h"
 #include "rtc_base/refcount.h"
@@ -68,7 +68,7 @@ class RtpTransceiverInterface : public rtc::RefCountInterface {
   // remote descriptions. Before negotiation is complete, the mid value may be
   // null. After rollbacks, the value may change from a non-null value to null.
   // https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver-mid
-  virtual rtc::Optional<std::string> mid() const = 0;
+  virtual absl::optional<std::string> mid() const = 0;
 
   // The sender attribute exposes the RtpSender corresponding to the RTP media
   // that may be sent with the transceiver's mid. The sender is always present,
@@ -105,7 +105,7 @@ class RtpTransceiverInterface : public rtc::RefCountInterface {
   // for this transceiver. If this transceiver has never been represented in an
   // offer/answer exchange, or if the transceiver is stopped, the value is null.
   // https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver-currentdirection
-  virtual rtc::Optional<RtpTransceiverDirection> current_direction() const = 0;
+  virtual absl::optional<RtpTransceiverDirection> current_direction() const = 0;
 
   // The Stop method irreversibly stops the RtpTransceiver. The sender of this
   // transceiver will no longer send, the receiver will no longer receive.