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}

webrtc/base/thread_checker_impl.cc

@@ -12,6 +12,8 @@
 
 #include "webrtc/base/thread_checker_impl.h"
 
+#include "webrtc/base/platform_thread.h"
+
 namespace rtc {
 
 ThreadCheckerImpl::ThreadCheckerImpl() : valid_thread_(CurrentThreadRef()) {