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.cc b/modules/audio_coding/test/RTPFile.cc
index 38f304e..6ea5354 100644
--- a/modules/audio_coding/test/RTPFile.cc
+++ b/modules/audio_coding/test/RTPFile.cc
@@ -20,7 +20,7 @@
 #endif
 
 #include "audio_coding_module.h"
-#include "system_wrappers/include/rw_lock_wrapper.h"
+#include "rtc_base/synchronization/rw_lock_wrapper.h"
 // TODO(tlegrand): Consider removing usage of gtest.
 #include "test/gtest.h"
 #include "typedefs.h"  // NOLINT(build/include)