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}

api/audio_codecs/ilbc/audio_decoder_ilbc.cc

@@ -19,12 +19,12 @@
 
 namespace webrtc {
 
-rtc::Optional<AudioDecoderIlbc::Config> AudioDecoderIlbc::SdpToConfig(
+absl::optional<AudioDecoderIlbc::Config> AudioDecoderIlbc::SdpToConfig(
     const SdpAudioFormat& format) {
   return STR_CASE_CMP(format.name.c_str(), "ILBC") == 0 &&
                  format.clockrate_hz == 8000 && format.num_channels == 1
-             ? rtc::Optional<Config>(Config())
-             : rtc::nullopt;
+             ? absl::optional<Config>(Config())
+             : absl::nullopt;
 }
 
 void AudioDecoderIlbc::AppendSupportedDecoders(
@@ -34,7 +34,7 @@ void AudioDecoderIlbc::AppendSupportedDecoders(
 
 std::unique_ptr<AudioDecoder> AudioDecoderIlbc::MakeAudioDecoder(
     Config config,
-    rtc::Optional<AudioCodecPairId> /*codec_pair_id*/) {
+    absl::optional<AudioCodecPairId> /*codec_pair_id*/) {
   return rtc::MakeUnique<AudioDecoderIlbcImpl>();
 }