Revert of Relanding: Fixing crash that can occur if signal is modified while firing. (patchset #4 id:60001 of https://codereview.webrtc.org/2846593005/ )

Reason for revert:
Breaks XmppConnectionTest in Chrome.

https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_tsan_rel_ng/builds/65096

Original issue's description:
> Relanding: Fixing crash that can occur if signal is modified while firing.
>
> The crash occurs if a slot causes the very next slot in iteration order
> to be disconnected.
>
> Relanding after fixing a race condition that this CL revealed. Previously
> the race resulted in an invalidated iterator, but now it will result in the
> iterator being modified, so TSan catches it.
>
> BUG=webrtc:7527
>
> Review-Url: https://codereview.webrtc.org/2846593005
> Cr-Original-Commit-Position: refs/heads/master@{#17943}
> Committed: 961c2adf1e
> Review-Url: https://codereview.webrtc.org/2846593005
> Cr-Commit-Position: refs/heads/master@{#17965}
> Committed: fc1af01557

TBR=pthatcher@webrtc.org,deadbeef@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7527

Review-Url: https://codereview.webrtc.org/2859443002
Cr-Commit-Position: refs/heads/master@{#17975}

webrtc/base/asyncinvoker.h

@@ -144,6 +144,9 @@ class AsyncInvoker : public MessageHandler {
   // 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,