Relanding: Use std::unique_ptr instead of rtc::scoped_refptr in AsyncInvoker.

The AsyncClosures only ever have one thing referencing them, so they
should be using std::unique_ptr to manage ownership. Maybe this code was
written before std::unique_ptr was available.

Originally reverted because it made a change to ScopedMessageData
that wasn't backwards compatible, and applications using the rtc::Thread
infrastructure may be using it.

BUG=None
NOTRY=True

Review-Url: https://codereview.webrtc.org/2689233003
Cr-Commit-Position: refs/heads/master@{#16684}

webrtc/base/asyncinvoker-inl.h

@@ -15,8 +15,6 @@
 #include "webrtc/base/callback.h"
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/messagehandler.h"
-#include "webrtc/base/refcount.h"
-#include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/base/sigslot.h"
 #include "webrtc/base/thread.h"
 
@@ -25,15 +23,13 @@ namespace rtc {
 class AsyncInvoker;
 
 // Helper class for AsyncInvoker. Runs a task and triggers a callback
-// on the calling thread if necessary. Instances are ref-counted so their
-// lifetime can be independent of AsyncInvoker.
-class AsyncClosure : public RefCountInterface {
+// on the calling thread if necessary.
+class AsyncClosure {
  public:
+  virtual ~AsyncClosure() {}
   // Runs the asynchronous task, and triggers a callback to the calling
   // thread if needed. Should be called from the target thread.
   virtual void Execute() = 0;
- protected:
-  ~AsyncClosure() override {}
 };
 
 // Simple closure that doesn't trigger a callback for the calling thread.