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/neteq_impl_unittest.cc b/modules/audio_coding/neteq/neteq_impl_unittest.cc
index cade064..2a21e51 100644
--- a/modules/audio_coding/neteq/neteq_impl_unittest.cc
+++ b/modules/audio_coding/neteq/neteq_impl_unittest.cc
@@ -348,9 +348,8 @@
       .Times(1);
   EXPECT_CALL(*mock_packet_buffer_, InsertPacketList(_, _, _, _, _))
       .Times(2)
-      .WillRepeatedly(
-          DoAll(SetArgPointee<2>(rtc::Optional<uint8_t>(kPayloadType)),
-                WithArg<0>(Invoke(DeletePacketsAndReturnOk))));
+      .WillRepeatedly(DoAll(SetArgPointee<2>(kPayloadType),
+                            WithArg<0>(Invoke(DeletePacketsAndReturnOk))));
   // SetArgPointee<2>(kPayloadType) means that the third argument (zero-based
   // index) is a pointer, and the variable pointed to is set to kPayloadType.
   // Also invoke the function DeletePacketsAndReturnOk to properly delete all