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/test/PCMFile.cc b/modules/audio_coding/test/PCMFile.cc
index 73c8542..e9e9430 100644
--- a/modules/audio_coding/test/PCMFile.cc
+++ b/modules/audio_coding/test/PCMFile.cc
@@ -225,7 +225,7 @@
 }
 
 void PCMFile::SetNum10MsBlocksToRead(int value) {
-  num_10ms_blocks_to_read_ = rtc::Optional<int>(value);
+  num_10ms_blocks_to_read_ = value;
 }
 
 }  // namespace webrtc