Replace rtc::Optional with absl::optional in api

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

This CL generated by running script with parameter 'api'
Then undo changes to optional target itself and optional_unittests

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: I44093da213369d6a502e33792c694f620f53b779
Reviewed-on: https://webrtc-review.googlesource.com/84621
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23707}
diff --git a/api/audio_codecs/builtin_audio_decoder_factory.cc b/api/audio_codecs/builtin_audio_decoder_factory.cc
index c3e5d50..e3ca1b0 100644
--- a/api/audio_codecs/builtin_audio_decoder_factory.cc
+++ b/api/audio_codecs/builtin_audio_decoder_factory.cc
@@ -33,7 +33,8 @@
 template <typename T>
 struct NotAdvertised {
   using Config = typename T::Config;
-  static rtc::Optional<Config> SdpToConfig(const SdpAudioFormat& audio_format) {
+  static absl::optional<Config> SdpToConfig(
+      const SdpAudioFormat& audio_format) {
     return T::SdpToConfig(audio_format);
   }
   static void AppendSupportedDecoders(std::vector<AudioCodecSpec>* specs) {
@@ -41,7 +42,7 @@
   }
   static std::unique_ptr<AudioDecoder> MakeAudioDecoder(
       const Config& config,
-      rtc::Optional<AudioCodecPairId> codec_pair_id = rtc::nullopt) {
+      absl::optional<AudioCodecPairId> codec_pair_id = absl::nullopt) {
     return T::MakeAudioDecoder(config, codec_pair_id);
   }
 };