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/modules/utility/source/process_thread_impl.cc b/webrtc/modules/utility/source/process_thread_impl.cc
index 9c739de..63e77d9 100644
--- a/webrtc/modules/utility/source/process_thread_impl.cc
+++ b/webrtc/modules/utility/source/process_thread_impl.cc
@@ -76,8 +76,8 @@
       m.module->ProcessThreadAttached(this);
   }
 
-  thread_ = ThreadWrapper::CreateThread(&ProcessThreadImpl::Run, this,
-                                        thread_name_);
+  thread_ =
+      PlatformThread::CreateThread(&ProcessThreadImpl::Run, this, thread_name_);
   RTC_CHECK(thread_->Start());
 }