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.h b/api/audio_codecs/audio_decoder.h
index 021288f..4852ad7 100644
--- a/api/audio_codecs/audio_decoder.h
+++ b/api/audio_codecs/audio_decoder.h
@@ -14,8 +14,8 @@
 #include <memory>
 #include <vector>
 
+#include "absl/types/optional.h"
 #include "api/array_view.h"
-#include "api/optional.h"
 #include "rtc_base/buffer.h"
 #include "rtc_base/constructormagic.h"
 #include "typedefs.h"  // NOLINT(build/include)
@@ -53,11 +53,11 @@
 
     // 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 rtc::Optional containing the
+    // call to Duration() . On success, returns an absl::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
-    // rtc::Optional. Decode may be called at most once per frame object.
-    virtual rtc::Optional<DecodeResult> Decode(
+    // absl::optional. Decode may be called at most once per frame object.
+    virtual absl::optional<DecodeResult> Decode(
         rtc::ArrayView<int16_t> decoded) const = 0;
   };