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/ortc/mediadescription.h

@@ -15,8 +15,8 @@
 #include <utility>
 #include <vector>
 
+#include "absl/types/optional.h"
 #include "api/cryptoparams.h"
-#include "api/optional.h"
 
 namespace webrtc {
 
@@ -31,7 +31,7 @@ class MediaDescription {
   // The mid(media stream identification) is used for identifying media streams
   // within a session description.
   // https://tools.ietf.org/html/rfc5888#section-6
-  rtc::Optional<std::string> mid() const { return mid_; }
+  absl::optional<std::string> mid() const { return mid_; }
   void set_mid(std::string mid) { mid_.emplace(std::move(mid)); }
 
   // Security keys and parameters for this media stream. Can be used to
@@ -43,7 +43,7 @@ class MediaDescription {
   }
 
  private:
-  rtc::Optional<std::string> mid_;
+  absl::optional<std::string> mid_;
 
   std::vector<cricket::CryptoParams> sdes_params_;
 };

api/ortc/rtptransportinterface.h

@@ -13,7 +13,7 @@
 
 #include <string>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 #include "api/ortc/packettransportinterface.h"
 #include "api/rtcerror.h"
 #include "api/rtp_headers.h"