Remove incompatiblities with absl::optional in audio_coding

PCMFile.cc uses RTC_DCHECK. include and depend on rtc_base:checks target directly

change usage of value_or by using explicit constructor instead of implicit

Bug: webrtc:9078
Change-Id: I63c596b8a05b387e56df846b15c33a605fbad4e6
Reviewed-on: https://webrtc-review.googlesource.com/69985
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22897}

modules/audio_coding/test/PCMFile.cc

@@ -14,6 +14,7 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "rtc_base/checks.h"
 #include "test/gtest.h"
 
 namespace webrtc {