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

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

This CL generated by running script with parameter 'pc'

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: Ide3b9eb32df7f25991f898ac58fcb119c9f8ae12
Reviewed-on: https://webrtc-review.googlesource.com/84181
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23669}
diff --git a/pc/rtcstatscollector.h b/pc/rtcstatscollector.h
index bad281f..43d5653 100644
--- a/pc/rtcstatscollector.h
+++ b/pc/rtcstatscollector.h
@@ -17,7 +17,7 @@
 #include <string>
 #include <vector>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 #include "api/stats/rtcstats_objects.h"
 #include "api/stats/rtcstatscollectorcallback.h"
 #include "api/stats/rtcstatsreport.h"
@@ -145,8 +145,8 @@
   struct RtpTransceiverStatsInfo {
     rtc::scoped_refptr<RtpTransceiver> transceiver;
     cricket::MediaType media_type;
-    rtc::Optional<std::string> mid;
-    rtc::Optional<std::string> transport_name;
+    absl::optional<std::string> mid;
+    absl::optional<std::string> transport_name;
     std::unique_ptr<TrackMediaInfoMap> track_media_info_map;
   };