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.cc b/webrtc/modules/utility/source/process_thread_impl.cc
index 63e77d9..1da4225 100644
--- a/webrtc/modules/utility/source/process_thread_impl.cc
+++ b/webrtc/modules/utility/source/process_thread_impl.cc
@@ -76,9 +76,9 @@
       m.module->ProcessThreadAttached(this);
   }
 
-  thread_ =
-      PlatformThread::CreateThread(&ProcessThreadImpl::Run, this, thread_name_);
-  RTC_CHECK(thread_->Start());
+  thread_.reset(
+      new rtc::PlatformThread(&ProcessThreadImpl::Run, this, thread_name_));
+  thread_->Start();
 }
 
 void ProcessThreadImpl::Stop() {
@@ -93,7 +93,7 @@
 
   wake_up_->Set();
 
-  RTC_CHECK(thread_->Stop());
+  thread_->Stop();
   stop_ = false;
 
   // TODO(tommi): Since DeRegisterModule is currently being called from