Replace rtc::Optional with absl::optional in modules/audio_coding

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

This CL generated by running script with parameter 'modules/audio_coding'

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: Ic980ee605148fdb160666d4aa03cc87175e48fe8
Reviewed-on: https://webrtc-review.googlesource.com/84130
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23659}

modules/audio_coding/acm2/audio_coding_module_unittest.cc

@@ -241,7 +241,7 @@ class AudioCodingModuleTestOldApi : public ::testing::Test {
 
   // These two have to be kept in sync for now. In the future, we'll be able to
   // eliminate the CodecInst and keep only the SdpAudioFormat.
-  rtc::Optional<SdpAudioFormat> audio_format_;
+  absl::optional<SdpAudioFormat> audio_format_;
   CodecInst codec_;
 
   Clock* clock_;
@@ -1058,7 +1058,7 @@ TEST_F(AcmReceiverBitExactnessOldApi, 48kHzOutputExternalDecoder) {
     }
     std::unique_ptr<AudioDecoder> MakeAudioDecoder(
         const SdpAudioFormat& format,
-        rtc::Optional<AudioCodecPairId> codec_pair_id) override {
+        absl::optional<AudioCodecPairId> codec_pair_id) override {
       return format.name == "MockPCMu"
                  ? std::move(mock_decoder_)
                  : fact_->MakeAudioDecoder(format, codec_pair_id);