Migrate absl::optional to std::optional

Bug: webrtc:342905193
No-Try: True
Change-Id: Icc968be43b8830038ea9a1f5f604307220457807
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/361021
Auto-Submit: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42911}

api/audio_codecs/audio_decoder.h

diff --git a/api/audio_codecs/audio_decoder.h b/api/audio_codecs/audio_decoder.h
index 4113874..21d6056 100644
--- a/api/audio_codecs/audio_decoder.h
+++ b/api/audio_codecs/audio_decoder.h
@@ -15,9 +15,9 @@
 #include <stdint.h>
 
 #include <memory>
+#include <optional>
 #include <vector>
 
-#include "absl/types/optional.h"
 #include "api/array_view.h"
 #include "rtc_base/buffer.h"
 
@@ -57,11 +57,11 @@
 
     // Decodes this frame of audio and writes the result in `decoded`.
     // `decoded` must be large enough to store as many samples as indicated by a
-    // call to Duration() . On success, returns an absl::optional containing the
+    // call to Duration() . On success, returns an std::optional containing the
     // total number of samples across all channels, as well as whether the
     // decoder produced comfort noise or speech. On failure, returns an empty
-    // absl::optional. Decode may be called at most once per frame object.
-    virtual absl::optional<DecodeResult> Decode(
+    // std::optional. Decode may be called at most once per frame object.
+    virtual std::optional<DecodeResult> Decode(
         rtc::ArrayView<int16_t> decoded) const = 0;
   };