Replace scoped_ptr with unique_ptr everywhere

But keep #including scoped_ptr.h in .h files, so as not to break
WebRTC users who expect those .h files to give them rtc::scoped_ptr.

BUG=webrtc:5520

Review-Url: https://codereview.webrtc.org/1937693002
Cr-Commit-Position: refs/heads/master@{#12581}
diff --git a/webrtc/system_wrappers/source/file_impl.h b/webrtc/system_wrappers/source/file_impl.h
index 06ba582..8764f72 100644
--- a/webrtc/system_wrappers/source/file_impl.h
+++ b/webrtc/system_wrappers/source/file_impl.h
@@ -13,6 +13,8 @@
 
 #include <stdio.h>
 
+#include <memory>
+
 #include "webrtc/base/scoped_ptr.h"
 #include "webrtc/system_wrappers/include/file_wrapper.h"
 
@@ -52,7 +54,7 @@
   int CloseFileImpl();
   int FlushImpl();
 
-  rtc::scoped_ptr<RWLockWrapper> rw_lock_;
+  std::unique_ptr<RWLockWrapper> rw_lock_;
 
   FILE* id_;
   bool managed_file_handle_;