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/audio_decoder_unittest.cc b/modules/audio_coding/neteq/audio_decoder_unittest.cc
index ae164fd..3181d6f 100644
--- a/modules/audio_coding/neteq/audio_decoder_unittest.cc
+++ b/modules/audio_coding/neteq/audio_decoder_unittest.cc
@@ -462,7 +462,7 @@
 
 namespace {
 int SetAndGetTargetBitrate(AudioEncoder* audio_encoder, int rate) {
-  audio_encoder->OnReceivedUplinkBandwidth(rate, rtc::Optional<int64_t>());
+  audio_encoder->OnReceivedUplinkBandwidth(rate, rtc::nullopt);
   return audio_encoder->GetTargetBitrate();
 }
 void TestSetAndGetTargetBitratesWithFixedCodec(AudioEncoder* audio_encoder,