Relanding #2: Fixing crash that can occur if signal is modified while firing.

webrtc/base/asyncinvoker.h

 /*
  *  Copyright 2014 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 126 matching lines @@
     DoInvoke(posted_from, thread, std::move(closure), id);
   }
 
   // Synchronously execute on |thread| all outstanding calls we own
   // that are pending on |thread|, and wait for calls to complete
   // before returning. Optionally filter by message id.
   // 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;
   void DoInvoke(const Location& posted_from,
                 Thread* thread,
                 std::unique_ptr<AsyncClosure> closure,
                 uint32_t id);
   void DoInvokeDelayed(const Location& posted_from,
                        Thread* thread,
                        std::unique_ptr<AsyncClosure> closure,
                        uint32_t delay_ms,
@@ skipping 92 matching lines @@
   void ThreadDestroyed();
 
   CriticalSection crit_;
   Thread* thread_ GUARDED_BY(crit_);
   AsyncInvoker invoker_ GUARDED_BY(crit_);
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_ASYNCINVOKER_H_