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.h b/webrtc/modules/audio_coding/neteq/neteq_impl.h
index 514fdaa..737efd4 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.h
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.h
@@ -160,7 +160,7 @@
   // Disables post-decode VAD.
   void DisableVad() override;
 
-  bool GetPlayoutTimestamp(uint32_t* timestamp) override;
+  rtc::Optional<uint32_t> GetPlayoutTimestamp() override;
 
   int last_output_sample_rate_hz() const override;