Move rw_lock_wrapper.h to rtc_base/synchronization/

This moves it from an API directory (system_wrappers/include/) to a
non-API directory, which is exactly what we want for utilities like
this.

BUG=webrtc:8445
NOPRESUBMIT=true

Change-Id: Ie2879aca5fc1667e4222499d2a8fc2bba9ae2425
Reviewed-on: https://webrtc-review.googlesource.com/21328
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22587}
diff --git a/modules/audio_coding/test/RTPFile.h b/modules/audio_coding/test/RTPFile.h
index 34dae63..d7e8c26 100644
--- a/modules/audio_coding/test/RTPFile.h
+++ b/modules/audio_coding/test/RTPFile.h
@@ -16,7 +16,7 @@
 
 #include "modules/audio_coding/include/audio_coding_module.h"
 #include "modules/include/module_common_types.h"
-#include "system_wrappers/include/rw_lock_wrapper.h"
+#include "rtc_base/synchronization/rw_lock_wrapper.h"
 #include "typedefs.h"  // NOLINT(build/include)
 
 namespace webrtc {