Replace rtc::Optional with absl::optional in audio, call and video

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

This CL generated by running script with parameters 'audio call video':
#!/bin/bash
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: I02c5db956846a88a268a300ba086703a02d62e36
Reviewed-on: https://webrtc-review.googlesource.com/83722
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23628}
diff --git a/video/send_statistics_proxy.h b/video/send_statistics_proxy.h
index bef3b28..7239e7f 100644
--- a/video/send_statistics_proxy.h
+++ b/video/send_statistics_proxy.h
@@ -162,7 +162,7 @@
     bool is_active = false;
     int on_off_events = 0;
     int64_t elapsed_ms = 0;
-    rtc::Optional<int64_t> last_update_ms;
+    absl::optional<int64_t> last_update_ms;
     const int max_frame_diff_ms = 2000;
   };
   struct FallbackEncoderInfoDisabled {
@@ -235,8 +235,8 @@
   Clock* const clock_;
   const std::string payload_name_;
   const VideoSendStream::Config::Rtp rtp_config_;
-  const rtc::Optional<int> fallback_max_pixels_;
-  const rtc::Optional<int> fallback_max_pixels_disabled_;
+  const absl::optional<int> fallback_max_pixels_;
+  const absl::optional<int> fallback_max_pixels_disabled_;
   rtc::CriticalSection crit_;
   VideoEncoderConfig::ContentType content_type_ RTC_GUARDED_BY(crit_);
   const int64_t start_ms_;
@@ -248,7 +248,7 @@
   rtc::RateTracker media_byte_rate_tracker_ RTC_GUARDED_BY(crit_);
   rtc::RateTracker encoded_frame_rate_tracker_ RTC_GUARDED_BY(crit_);
 
-  rtc::Optional<int64_t> last_outlier_timestamp_ RTC_GUARDED_BY(crit_);
+  absl::optional<int64_t> last_outlier_timestamp_ RTC_GUARDED_BY(crit_);
 
   // Contains stats used for UMA histograms. These stats will be reset if
   // content type changes between real-time video and screenshare, since these