Optional: Use nullopt and implicit construction in /modules/audio_coding

Changes places where we explicitly construct an Optional to instead use
nullopt or the requisite value type only.

This CL was uploaded by git cl split.

R=kwiberg@webrtc.org

Bug: None
Change-Id: I055411a3e521964c81100869a197dd92f5608f1b
Reviewed-on: https://webrtc-review.googlesource.com/23619
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20728}
diff --git a/modules/audio_coding/neteq/neteq_impl.cc b/modules/audio_coding/neteq/neteq_impl.cc
index 089f6ca..14f2fc7 100644
--- a/modules/audio_coding/neteq/neteq_impl.cc
+++ b/modules/audio_coding/neteq/neteq_impl.cc
@@ -423,10 +423,9 @@
     // We don't have a valid RTP timestamp until we have decoded our first
     // RTP packet. Also, the RTP timestamp is not accurate while playing CNG,
     // which is indicated by returning an empty value.
-    return rtc::Optional<uint32_t>();
+    return rtc::nullopt;
   }
-  return rtc::Optional<uint32_t>(
-      timestamp_scaler_->ToExternal(playout_timestamp_));
+  return timestamp_scaler_->ToExternal(playout_timestamp_);
 }
 
 int NetEqImpl::last_output_sample_rate_hz() const {
@@ -439,7 +438,7 @@
   const DecoderDatabase::DecoderInfo* di =
       decoder_database_->GetDecoderInfo(payload_type);
   if (!di) {
-    return rtc::Optional<CodecInst>();
+    return rtc::nullopt;
   }
 
   // Create a CodecInst with some fields set. The remaining fields are zeroed,
@@ -452,7 +451,7 @@
   ci.plfreq = di->IsRed() ? 8000 : di->SampleRateHz();
   AudioDecoder* const decoder = di->GetDecoder();
   ci.channels = decoder ? decoder->Channels() : 1;
-  return rtc::Optional<CodecInst>(ci);
+  return ci;
 }
 
 rtc::Optional<SdpAudioFormat> NetEqImpl::GetDecoderFormat(
@@ -461,9 +460,9 @@
   const DecoderDatabase::DecoderInfo* const di =
       decoder_database_->GetDecoderInfo(payload_type);
   if (!di) {
-    return rtc::Optional<SdpAudioFormat>();  // Payload type not registered.
+    return rtc::nullopt;  // Payload type not registered.
   }
-  return rtc::Optional<SdpAudioFormat>(di->GetFormat());
+  return di->GetFormat();
 }
 
 int NetEqImpl::SetTargetNumberOfChannels() {
@@ -2004,7 +2003,7 @@
     stats_.JitterBufferDelay(extracted_samples, waiting_time_ms);
 
     packet_list->push_back(std::move(*packet));  // Store packet in list.
-    packet = rtc::Optional<Packet>();  // Ensure it's never used after the move.
+    packet = rtc::nullopt;  // Ensure it's never used after the move.
 
     // Check what packet is available next.
     next_packet = packet_buffer_->PeekNextPacket();