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 from modules with parameters
'pacing video_coding congestion_controller remote_bitrate_estimator':

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: I8ea501d7f1ee36e8d8cd3ed37e6b763c7fe29118
Reviewed-on: https://webrtc-review.googlesource.com/83900
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23640}
diff --git a/modules/video_coding/timing.cc b/modules/video_coding/timing.cc
index 40b8340..310467b 100644
--- a/modules/video_coding/timing.cc
+++ b/modules/video_coding/timing.cc
@@ -239,7 +239,7 @@
   timing_frame_info_.emplace(info);
 }
 
-rtc::Optional<TimingFrameInfo> VCMTiming::GetTimingFrameInfo() {
+absl::optional<TimingFrameInfo> VCMTiming::GetTimingFrameInfo() {
   rtc::CritScope cs(&crit_sect_);
   return timing_frame_info_;
 }