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}

webrtc/modules/utility/include/process_thread.h

@@ -15,23 +15,17 @@
 
 #include "webrtc/typedefs.h"
 
-#if defined(WEBRTC_WIN)
-// Due to a bug in the std::unique_ptr implementation that ships with MSVS,
-// we need the full definition of QueuedTask, on Windows.
-#include "webrtc/base/task_queue.h"
-#else
-namespace rtc {
-class QueuedTask;
-}
-#endif
-
 namespace webrtc {
 class Module;
 
-// TODO(tommi): ProcessThread probably doesn't need to be a virtual
-// interface.  There exists one override besides ProcessThreadImpl,
-// MockProcessThread, but when looking at how it is used, it seems
-// a nullptr might suffice (or simply an actual ProcessThread instance).
+class ProcessTask {
+ public:
+  ProcessTask() {}
+  virtual ~ProcessTask() {}
+
+  virtual void Run() = 0;
+};
+
 class ProcessThread {
  public:
   virtual ~ProcessThread();
@@ -57,7 +51,7 @@ class ProcessThread {
   // construction thread of the ProcessThread instance, if the task did not
   // get a chance to run (e.g. posting the task while shutting down or when
   // the thread never runs).
-  virtual void PostTask(std::unique_ptr<rtc::QueuedTask> task) = 0;
+  virtual void PostTask(std::unique_ptr<ProcessTask> task) = 0;
 
   // Adds a module that will start to receive callbacks on the worker thread.
   // Can be called from any thread.