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/quality_threshold.h b/video/quality_threshold.h
index 519e2c3..4fad555 100644
--- a/video/quality_threshold.h
+++ b/video/quality_threshold.h
@@ -13,7 +13,7 @@
 
 #include <memory>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 
 namespace webrtc {
 
@@ -27,9 +27,9 @@
                    int max_measurements);
 
   void AddMeasurement(int measurement);
-  rtc::Optional<bool> IsHigh() const;
-  rtc::Optional<double> CalculateVariance() const;
-  rtc::Optional<double> FractionHigh(int min_required_samples) const;
+  absl::optional<bool> IsHigh() const;
+  absl::optional<double> CalculateVariance() const;
+  absl::optional<double> FractionHigh(int min_required_samples) const;
 
  private:
   const std::unique_ptr<int[]> buffer_;
@@ -39,7 +39,7 @@
   const int high_threshold_;
   int until_full_;
   int next_index_;
-  rtc::Optional<bool> is_high_;
+  absl::optional<bool> is_high_;
   int sum_;
   int count_low_;
   int count_high_;