Change NetEq::GetPlayoutTimestamp to return an rtc::Optional<uint32_t>

This is in preparation for changes to when the playout timestamp is
valid.

BUG=webrtc:5669

Review URL: https://codereview.webrtc.org/1853183002

Cr-Commit-Position: refs/heads/master@{#12256}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.cc b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
index aa97381..b4dfc99 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
@@ -401,15 +401,15 @@
   vad_->Disable();
 }
 
-bool NetEqImpl::GetPlayoutTimestamp(uint32_t* timestamp) {
+rtc::Optional<uint32_t> NetEqImpl::GetPlayoutTimestamp() {
   rtc::CritScope lock(&crit_sect_);
   if (first_packet_) {
     // We don't have a valid RTP timestamp until we have decoded our first
     // RTP packet.
-    return false;
+    return rtc::Optional<uint32_t>();
   }
-  *timestamp = timestamp_scaler_->ToExternal(playout_timestamp_);
-  return true;
+  return rtc::Optional<uint32_t>(
+      timestamp_scaler_->ToExternal(playout_timestamp_));
 }
 
 int NetEqImpl::last_output_sample_rate_hz() const {