Revert of Change ProcessThread's task type to be the one from TaskQueue. (patchset #3 id:80001 of https://codereview.webrtc.org/2016043003/ )

Reason for revert:
Downstream issues

Original issue's description:
> Change ProcessThread's task type to be the one from TaskQueue.
> ProcessThread will eventually be replaced by TaskQueue, so this is the first little step.
>
> BUG=
> R=magjed@webrtc.org
>
> Committed: https://crrev.com/400a276c8a0b299190ff17a81edd8780a26d63d3
> Cr-Commit-Position: refs/heads/master@{#12952}

TBR=magjed@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=

Review-Url: https://codereview.webrtc.org/2020783003
Cr-Commit-Position: refs/heads/master@{#12953}
diff --git a/webrtc/modules/utility/source/process_thread_impl.h b/webrtc/modules/utility/source/process_thread_impl.h
index 510ab52..330aec9 100644
--- a/webrtc/modules/utility/source/process_thread_impl.h
+++ b/webrtc/modules/utility/source/process_thread_impl.h
@@ -33,7 +33,7 @@
   void Stop() override;
 
   void WakeUp(Module* module) override;
-  void PostTask(std::unique_ptr<rtc::QueuedTask> task) override;
+  void PostTask(std::unique_ptr<ProcessTask> task) override;
 
   void RegisterModule(Module* module) override;
   void DeRegisterModule(Module* module) override;
@@ -75,7 +75,8 @@
   std::unique_ptr<rtc::PlatformThread> thread_;
 
   ModuleList modules_;
-  std::queue<rtc::QueuedTask*> queue_;
+  // TODO(tommi): Support delayed tasks.
+  std::queue<ProcessTask*> queue_;
   bool stop_;
   const char* thread_name_;
 };