Reland of Change ProcessThread's task type to be the one from TaskQueue. (patchset #1 id:1 of https://codereview.webrtc.org/2020783003/ )

Reason for revert:
Reland. It was a false alarm.

Original issue's description:
> 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=
>
> Committed: https://crrev.com/641455176a1241dea6dda7071ba4162f41a0b5fc
> Cr-Commit-Position: refs/heads/master@{#12953}

TBR=magjed@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review-Url: https://codereview.webrtc.org/2017333002
Cr-Commit-Position: refs/heads/master@{#12954}
diff --git a/webrtc/modules/utility/source/process_thread_impl.h b/webrtc/modules/utility/source/process_thread_impl.h
index 330aec9..510ab52 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<ProcessTask> task) override;
+  void PostTask(std::unique_ptr<rtc::QueuedTask> task) override;
 
   void RegisterModule(Module* module) override;
   void DeRegisterModule(Module* module) override;
@@ -75,8 +75,7 @@
   std::unique_ptr<rtc::PlatformThread> thread_;
 
   ModuleList modules_;
-  // TODO(tommi): Support delayed tasks.
-  std::queue<ProcessTask*> queue_;
+  std::queue<rtc::QueuedTask*> queue_;
   bool stop_;
   const char* thread_name_;
 };