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/audio_decoder.cc b/api/audio_codecs/audio_decoder.cc
index 4903fb6..00e45d9 100644
--- a/api/audio_codecs/audio_decoder.cc
+++ b/api/audio_codecs/audio_decoder.cc
@@ -33,14 +33,14 @@
     return ret < 0 ? 0 : static_cast<size_t>(ret);
   }
 
-  rtc::Optional<DecodeResult> Decode(
+  absl::optional<DecodeResult> Decode(
       rtc::ArrayView<int16_t> decoded) const override {
     auto speech_type = AudioDecoder::kSpeech;
     const int ret = decoder_->Decode(
         payload_.data(), payload_.size(), decoder_->SampleRateHz(),
         decoded.size() * sizeof(int16_t), decoded.data(), &speech_type);
-    return ret < 0 ? rtc::nullopt
-                   : rtc::Optional<DecodeResult>(
+    return ret < 0 ? absl::nullopt
+                   : absl::optional<DecodeResult>(
                          {static_cast<size_t>(ret), speech_type});
   }