[Sync] Signal UIModelWorker to abort on sync shutdown.

components/sync/base/scoped_event_signal.cc

 // Copyright 2016 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 "components/sync/base/scoped_event_signal.h"
 
 #include "base/logging.h"
 #include "base/synchronization/waitable_event.h"
 
 namespace syncer {
@@ skipping 13 matching lines @@
   event_ = other.event_;
   other.event_ = nullptr;
   return *this;
 }
 
 ScopedEventSignal::~ScopedEventSignal() {
   if (event_)
     event_->Signal();
 }
 
+bool ScopedEventSignal::IsSignaled() const {
+  return event_->IsSignaled();
+}
+
 }  // namespace syncer