Replace scoped_ptr with unique_ptr in webrtc/base/

This propagated into various other places. Also had to #include headers that
were implicitly pulled by "scoped_ptr.h".

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1920043002

Cr-Commit-Position: refs/heads/master@{#12501}

webrtc/base/platform_thread_unittest.cc

@@ -11,7 +11,6 @@
 #include "webrtc/base/platform_thread.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
-#include "webrtc/base/scoped_ptr.h"
 #include "webrtc/system_wrappers/include/sleep.h"
 
 namespace rtc {