Move ThreadWrapper to ProcessThread in base.

Also removes all virtual methods. Permits using a thread from
rtc_base_approved (namely event tracing).

BUG=webrtc:5158
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1469013002

Cr-Commit-Position: refs/heads/master@{#10760}
diff --git a/webrtc/system_wrappers/source/trace_impl.h b/webrtc/system_wrappers/source/trace_impl.h
index ed49d9d..c6d81d5 100644
--- a/webrtc/system_wrappers/source/trace_impl.h
+++ b/webrtc/system_wrappers/source/trace_impl.h
@@ -16,7 +16,7 @@
 #include "webrtc/system_wrappers/include/event_wrapper.h"
 #include "webrtc/system_wrappers/include/file_wrapper.h"
 #include "webrtc/system_wrappers/include/static_instance.h"
-#include "webrtc/system_wrappers/include/thread_wrapper.h"
+#include "webrtc/base/platform_thread.h"
 #include "webrtc/system_wrappers/include/trace.h"
 
 namespace webrtc {