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}
diff --git a/modules/audio_processing/gain_control_impl.h b/modules/audio_processing/gain_control_impl.h
index 2674506..959422f 100644
--- a/modules/audio_processing/gain_control_impl.h
+++ b/modules/audio_processing/gain_control_impl.h
@@ -86,8 +86,8 @@
 
   std::vector<std::unique_ptr<GainController>> gain_controllers_;
 
-  rtc::Optional<size_t> num_proc_channels_ RTC_GUARDED_BY(crit_capture_);
-  rtc::Optional<int> sample_rate_hz_ RTC_GUARDED_BY(crit_capture_);
+  absl::optional<size_t> num_proc_channels_ RTC_GUARDED_BY(crit_capture_);
+  absl::optional<int> sample_rate_hz_ RTC_GUARDED_BY(crit_capture_);
 
   static int instance_counter_;
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(GainControlImpl);