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_unittest.cc b/video/send_statistics_proxy_unittest.cc
index d9e8343..5730317 100644
--- a/video/send_statistics_proxy_unittest.cc
+++ b/video/send_statistics_proxy_unittest.cc
@@ -331,7 +331,7 @@
 TEST_F(SendStatisticsProxyTest, OnSendEncodedImageIncreasesQpSum) {
   EncodedImage encoded_image;
   CodecSpecificInfo codec_info;
-  EXPECT_EQ(rtc::nullopt, statistics_proxy_->GetStats().qp_sum);
+  EXPECT_EQ(absl::nullopt, statistics_proxy_->GetStats().qp_sum);
   encoded_image.qp_ = 3;
   statistics_proxy_->OnSendEncodedImage(encoded_image, &codec_info);
   EXPECT_EQ(3u, statistics_proxy_->GetStats().qp_sum);
@@ -344,9 +344,9 @@
   EncodedImage encoded_image;
   CodecSpecificInfo codec_info;
   encoded_image.qp_ = -1;
-  EXPECT_EQ(rtc::nullopt, statistics_proxy_->GetStats().qp_sum);
+  EXPECT_EQ(absl::nullopt, statistics_proxy_->GetStats().qp_sum);
   statistics_proxy_->OnSendEncodedImage(encoded_image, &codec_info);
-  EXPECT_EQ(rtc::nullopt, statistics_proxy_->GetStats().qp_sum);
+  EXPECT_EQ(absl::nullopt, statistics_proxy_->GetStats().qp_sum);
 }
 
 TEST_F(SendStatisticsProxyTest, GetCpuAdaptationStats) {