Clean up PlatformThread.

* Move PlatformThread to rtc::.
* Remove ::CreateThread factory method.
* Make non-scoped_ptr from a lot of invocations.
* Make Start/Stop void.
* Remove rtc::Thread priorities, which were unused and would collide.
* Add ::IsRunning() to PlatformThread.

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1476453002 .

Cr-Commit-Position: refs/heads/master@{#10812}
diff --git a/webrtc/modules/utility/source/process_thread_impl.h b/webrtc/modules/utility/source/process_thread_impl.h
index 8f58932..1c0a0cd 100644
--- a/webrtc/modules/utility/source/process_thread_impl.h
+++ b/webrtc/modules/utility/source/process_thread_impl.h
@@ -70,7 +70,8 @@
 
   rtc::ThreadChecker thread_checker_;
   const rtc::scoped_ptr<EventWrapper> wake_up_;
-  rtc::scoped_ptr<PlatformThread> thread_;
+  // TODO(pbos): Remove scoped_ptr and stop recreating the thread.
+  rtc::scoped_ptr<rtc::PlatformThread> thread_;
 
   ModuleList modules_;
   // TODO(tommi): Support delayed tasks.