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/trace_impl.h b/webrtc/system_wrappers/source/trace_impl.h
index c6d81d5..5e45976 100644
--- a/webrtc/system_wrappers/source/trace_impl.h
+++ b/webrtc/system_wrappers/source/trace_impl.h
@@ -11,6 +11,8 @@
 #ifndef WEBRTC_SYSTEM_WRAPPERS_SOURCE_TRACE_IMPL_H_
 #define WEBRTC_SYSTEM_WRAPPERS_SOURCE_TRACE_IMPL_H_
 
+#include <memory>
+
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/scoped_ptr.h"
 #include "webrtc/system_wrappers/include/event_wrapper.h"
@@ -97,7 +99,7 @@
   uint32_t row_count_text_ GUARDED_BY(crit_);
   uint32_t file_count_text_ GUARDED_BY(crit_);
 
-  const rtc::scoped_ptr<FileWrapper> trace_file_ GUARDED_BY(crit_);
+  const std::unique_ptr<FileWrapper> trace_file_ GUARDED_BY(crit_);
   rtc::CriticalSection crit_;
 };