Replace rtc::Optional with absl::optional in modules/audio processing

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

This CL generated by running script with parameter 'modules/audio_processing'

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: Id29f8de59dba704787c2c38a3d05c60827c181b0
Reviewed-on: https://webrtc-review.googlesource.com/83982
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23653}

modules/audio_processing/echo_detector/circular_buffer.cc

@@ -28,9 +28,9 @@ void CircularBuffer::Push(float value) {
   RTC_DCHECK_LE(nr_elements_in_buffer_, buffer_.size());
 }
 
-rtc::Optional<float> CircularBuffer::Pop() {
+absl::optional<float> CircularBuffer::Pop() {
   if (nr_elements_in_buffer_ == 0) {
-    return rtc::nullopt;
+    return absl::nullopt;
   }
   const size_t index =
       (buffer_.size() + next_insertion_index_ - nr_elements_in_buffer_) %