Change AsyncInvoker to store its closure in a scoped_refptr instead of using a raw pointer.

This is just a cosmetic change and does not solve a particular bug.

R=henrika@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/38749004

Cr-Commit-Position: refs/heads/master@{#8194}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8194 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/base/asyncinvoker-inl.h

@@ -29,10 +29,11 @@ class AsyncInvoker;
 // lifetime can be independent of AsyncInvoker.
 class AsyncClosure : public RefCountInterface {
  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:
+  virtual ~AsyncClosure() {}
 };
 
 // Simple closure that doesn't trigger a callback for the calling thread.