Replace rtc::Optional with absl::optional

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

This CL generated by running script passing top level directories except rtc_base and api

find $@ -type f \( -name \*.h -o -name \*.cc -o -name \*.mm \) \
-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: I9465c172e65ba6e6ed4e4fdc35b0b265038d6f71
Reviewed-on: https://webrtc-review.googlesource.com/84584
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23697}

modules/BUILD.gn

@@ -37,7 +37,7 @@ rtc_source_set("module_api_public") {
   deps = [
     "..:webrtc_common",
     "../:typedefs",
-    "../api:optional",
+    "//third_party/abseil-cpp/absl/types:optional",
   ]
 }
 
@@ -53,13 +53,13 @@ rtc_source_set("module_api") {
     "..:webrtc_common",
     "../:typedefs",
     "../api:libjingle_peerconnection_api",
-    "../api:optional",
     "../api/transport:network_control",
     "../api/video:video_frame",
     "../api/video:video_frame_i420",
     "../rtc_base:deprecation",
     "../rtc_base:rtc_base_approved",
     "video_coding:codec_globals_headers",
+    "//third_party/abseil-cpp/absl/types:optional",
   ]
 }