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

webrtc/base/asyncinvoker.cc

Index: webrtc/base/asyncinvoker.cc
diff --git a/webrtc/base/asyncinvoker.cc b/webrtc/base/asyncinvoker.cc
index a143e25f19341bd2d3368c991aca8ab9c1e4d418..cabd8b506d0575ee3e3b41a479f6f7b9fcf63ee8 100644
--- a/webrtc/base/asyncinvoker.cc
+++ b/webrtc/base/asyncinvoker.cc
@@ -20,7 +20,6 @@ AsyncInvoker::AsyncInvoker() : invocation_complete_(false, false) {}
 
 AsyncInvoker::~AsyncInvoker() {
   destroying_ = true;
-  SignalInvokerDestroyed();
   // Messages for this need to be cleared *before* our destructor is complete.
   MessageQueueManager::Clear(this);
   // And we need to wait for any invocations that are still in progress on
@@ -126,8 +125,9 @@ NotifyingAsyncClosureBase::NotifyingAsyncClosureBase(
       calling_thread_(calling_thread) {
   calling_thread->SignalQueueDestroyed.connect(
       this, &NotifyingAsyncClosureBase::CancelCallback);
-  invoker->SignalInvokerDestroyed.connect(
-      this, &NotifyingAsyncClosureBase::CancelCallback);
+  // Note: We don't need to listen for the invoker being destroyed, because it
+  // will wait for this closure to be destroyed (and pending_invocations_ to go
+  // to 0) before its destructor completes.
 }
 
 NotifyingAsyncClosureBase::~NotifyingAsyncClosureBase() {