An assert was being hit because WaitableEvent's dtor was calling CloseHandle ...

base/waitable_event_win.cc

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/waitable_event.h"
 
 #include <math.h>
 #include <windows.h>
 
 #include "base/logging.h"
@@ skipping 10 matching lines @@
 
 WaitableEvent::WaitableEvent(HANDLE handle)
     : handle_(handle) {
   CHECK(handle) << "Tried to create WaitableEvent from NULL handle";
 }
 
 WaitableEvent::~WaitableEvent() {
   CloseHandle(handle_);
 }
 
+HANDLE WaitableEvent::Release() {
+  HANDLE rv = handle_;
+  handle_ = INVALID_HANDLE_VALUE;
+  return rv;
+}
+
 void WaitableEvent::Reset() {
   ResetEvent(handle_);
 }
 
 void WaitableEvent::Signal() {
   SetEvent(handle_);
 }
 
 bool WaitableEvent::IsSignaled() {
   return TimedWait(TimeDelta::FromMilliseconds(0));
@@ skipping 41 matching lines @@
                              INFINITE);  // no timeout
   if (result < WAIT_OBJECT_0 || result >= WAIT_OBJECT_0 + count) {
     NOTREACHED() << "WaitForMultipleObjects failed: " << GetLastError();
     return 0;
   }
 
   return result - WAIT_OBJECT_0;
 }
 
 }  // namespace base