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/end_to_end_tests/histogram_tests.cc b/video/end_to_end_tests/histogram_tests.cc
index c3a86b4..04255a8 100644
--- a/video/end_to_end_tests/histogram_tests.cc
+++ b/video/end_to_end_tests/histogram_tests.cc
@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 #include "modules/video_coding/codecs/vp8/include/vp8.h"
 #include "system_wrappers/include/metrics.h"
 #include "system_wrappers/include/metrics_default.h"
@@ -123,7 +123,7 @@
     const bool use_fec_;
     const bool screenshare_;
     test::FunctionVideoEncoderFactory encoder_factory_;
-    rtc::Optional<int64_t> start_runtime_ms_;
+    absl::optional<int64_t> start_runtime_ms_;
     int num_frames_received_ RTC_GUARDED_BY(&crit_);
   } test(use_rtx, use_fec, screenshare);