Add helper class GuardedAsyncInvoker to protect against thread dying

webrtc/base/asyncinvoker.cc

Index: webrtc/base/asyncinvoker.cc
diff --git a/webrtc/base/asyncinvoker.cc b/webrtc/base/asyncinvoker.cc
index 56f1a19d543bfeecc54b4ef7ffb7abbcb4400847..ee53e04184cf06cda4a674bcfec9bdf6e4a06666 100644
--- a/webrtc/base/asyncinvoker.cc
+++ b/webrtc/base/asyncinvoker.cc
@@ -10,6 +10,7 @@
 
 #include "webrtc/base/asyncinvoker.h"
 
+#include "webrtc/base/checks.h"
 #include "webrtc/base/logging.h"
 
 namespace rtc {
@@ -76,6 +77,29 @@ void AsyncInvoker::DoInvokeDelayed(Thread* thread,
                       new ScopedRefMessageData<AsyncClosure>(closure));
 }
 
+GuardedAsyncInvoker::GuardedAsyncInvoker() : thread_(Thread::Current()) {
+  thread_->SignalQueueDestroyed.connect(this,
+                                        &GuardedAsyncInvoker::ThreadDestroyed);
+}
+
+GuardedAsyncInvoker::~GuardedAsyncInvoker() {
+}
+
+bool GuardedAsyncInvoker::Flush(uint32 id) {
+  rtc::CritScope cs(&crit_);
+  if (thread_ == nullptr)
+    return false;
+  invoker_.Flush(thread_, id);
+  return true;
+}
+
+void GuardedAsyncInvoker::ThreadDestroyed() {
+  rtc::CritScope cs(&crit_);
+  // We should never get more than one notification about the thread dying.
+  DCHECK(thread_ != nullptr);
+  thread_ = nullptr;
+}
+
 NotifyingAsyncClosureBase::NotifyingAsyncClosureBase(AsyncInvoker* invoker,
                                                      Thread* calling_thread)
     : invoker_(invoker), calling_thread_(calling_thread) {