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/tools/neteq_input.h b/modules/audio_coding/neteq/tools/neteq_input.h
index 993ba04..88d9eb9 100644
--- a/modules/audio_coding/neteq/tools/neteq_input.h
+++ b/modules/audio_coding/neteq/tools/neteq_input.h
@@ -52,9 +52,9 @@
     const auto b = NextOutputEventTime();
     // Return the minimum of non-empty |a| and |b|, or empty if both are empty.
     if (a) {
-      return b ? rtc::Optional<int64_t>(std::min(*a, *b)) : a;
+      return b ? std::min(*a, *b) : a;
     }
-    return b ? b : rtc::Optional<int64_t>();
+    return b ? b : rtc::nullopt;
   }
 
   // Returns the next packet to be inserted into NetEq. The packet following the