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}

modules/video_coding/generic_encoder.h

@@ -79,8 +79,8 @@ class VCMEncodedFrameCallback : public EncodedImageCallback {
  private:
   // For non-internal-source encoders, returns encode started time and fixes
   // capture timestamp for the frame, if corrupted by the encoder.
-  rtc::Optional<int64_t> ExtractEncodeStartTime(size_t simulcast_svc_idx,
-                                                EncodedImage* encoded_image)
+  absl::optional<int64_t> ExtractEncodeStartTime(size_t simulcast_svc_idx,
+                                                 EncodedImage* encoded_image)
       RTC_EXCLUSIVE_LOCKS_REQUIRED(timing_params_lock_);
 
   void FillTimingInfo(size_t simulcast_svc_idx, EncodedImage* encoded_image);