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

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=henrik.lundin@webrtc.org

Bug: None
Change-Id: I733a83f702fe11884d229a1713cfac952727bde8
Reviewed-on: https://webrtc-review.googlesource.com/23601
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20786}
diff --git a/modules/audio_processing/test/debug_dump_replayer.cc b/modules/audio_processing/test/debug_dump_replayer.cc
index a3c4d63..a027d07 100644
--- a/modules/audio_processing/test/debug_dump_replayer.cc
+++ b/modules/audio_processing/test/debug_dump_replayer.cc
@@ -51,9 +51,9 @@
 // Get next event that has not run.
 rtc::Optional<audioproc::Event> DebugDumpReplayer::GetNextEvent() const {
   if (!has_next_event_)
-    return rtc::Optional<audioproc::Event>();
+    return rtc::nullopt;
   else
-    return rtc::Optional<audioproc::Event>(next_event_);
+    return next_event_;
 }
 
 // Run the next event. Returns the event type.