Revert of Relanding: Fixing crash that can occur if signal is modified while firing.

webrtc/base/asyncinvoker.h

Index: webrtc/base/asyncinvoker.h
diff --git a/webrtc/base/asyncinvoker.h b/webrtc/base/asyncinvoker.h
index 27f6a9fec0281bb37ae2c85ba16d76d063dd10d7..ed6124397460e35113e9093cc6fa76580ee33a7d 100644
--- a/webrtc/base/asyncinvoker.h
+++ b/webrtc/base/asyncinvoker.h
@@ -143,6 +143,9 @@
   // The destructor will not wait for outstanding calls, so if that
   // behavior is desired, call Flush() before destroying this object.
   void Flush(Thread* thread, uint32_t id = MQID_ANY);
+
+  // Signaled when this object is destructed.
+  sigslot::signal0<> SignalInvokerDestroyed;
 
  private:
   void OnMessage(Message* msg) override;