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/include/process_thread.h b/webrtc/modules/utility/include/process_thread.h
index f6913ea..8524a51 100644
--- a/webrtc/modules/utility/include/process_thread.h
+++ b/webrtc/modules/utility/include/process_thread.h
@@ -15,17 +15,23 @@
 
 #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;
 
-class ProcessTask {
- public:
-  ProcessTask() {}
-  virtual ~ProcessTask() {}
-
-  virtual void Run() = 0;
-};
-
+// 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 ProcessThread {
  public:
   virtual ~ProcessThread();
@@ -51,7 +57,7 @@
   // 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<ProcessTask> task) = 0;
+  virtual void PostTask(std::unique_ptr<rtc::QueuedTask> task) = 0;
 
   // Adds a module that will start to receive callbacks on the worker thread.
   // Can be called from any thread.