Replace rtc::Optional with absl::optional in modules/rtp_rtcp

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

This CL generated using script:
#!/bin/bash
dir=modules/rtp_rtcp
find $dir -type f \( -name \*.h -o -name \*.cc \) \
-exec sed -i 's|rtc::Optional|absl::optional|' {} \+ \
-exec sed -i 's|rtc::nullopt|absl::nullopt|' {} \+ \
-exec sed -i 's|#include "api/optional.h"|#include "absl/types/optional.h"|' {} \+

find $dir -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: Ife720849709959046329c1c9faa3f31aa13274dc
Reviewed-on: https://webrtc-review.googlesource.com/83584
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23624}

modules/rtp_rtcp/source/rtcp_transceiver_impl.cc

@@ -46,7 +46,7 @@ struct SenderReportTimes {
 
 struct RtcpTransceiverImpl::RemoteSenderState {
   uint8_t fir_sequence_number = 0;
-  rtc::Optional<SenderReportTimes> last_received_sender_report;
+  absl::optional<SenderReportTimes> last_received_sender_report;
   std::vector<MediaReceiverRtcpObserver*> observers;
 };
 
@@ -245,7 +245,7 @@ void RtcpTransceiverImpl::HandleSenderReport(
     return;
   RemoteSenderState& remote_sender =
       remote_senders_[sender_report.sender_ssrc()];
-  rtc::Optional<SenderReportTimes>& last =
+  absl::optional<SenderReportTimes>& last =
       remote_sender.last_received_sender_report;
   last.emplace();
   last->local_received_time_us = now_us;