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}

system_wrappers/source/rtp_to_ntp_estimator.cc

@@ -193,9 +193,9 @@ bool RtpToNtpEstimator::Estimate(int64_t rtp_timestamp,
   return true;
 }
 
-const rtc::Optional<RtpToNtpEstimator::Parameters> RtpToNtpEstimator::params()
+const absl::optional<RtpToNtpEstimator::Parameters> RtpToNtpEstimator::params()
     const {
-  rtc::Optional<Parameters> res;
+  absl::optional<Parameters> res;
   if (params_calculated_) {
     res.emplace(smoothing_filter_.GetFilteredValue());
   }