[Sync] Signal UIModelWorker to abort on sync shutdown.

components/sync/base/scoped_event_signal.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_BASE_SCOPED_EVENT_SIGNAL_H_
 #define COMPONENTS_SYNC_BASE_SCOPED_EVENT_SIGNAL_H_
 
 #include "base/macros.h"
 
 namespace base {
@@ skipping 11 matching lines @@
 
   // This ScopedEventSignal will signal |other|'s WaitableEvent in its
   // destructor. |other| will not signal anything in its destructor. The
   // assignment operator cannot be used if this ScopedEventSignal's
   // WaitableEvent hasn't been moved to another ScopedEventSignal.
   ScopedEventSignal(ScopedEventSignal&& other);
   ScopedEventSignal& operator=(ScopedEventSignal&& other);
 
   ~ScopedEventSignal();
 
+  // Whether the wrapped event has been signaled already.
+  bool IsSignaled() const;
+
  private:
   base::WaitableEvent* event_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedEventSignal);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_BASE_SCOPED_EVENT_SIGNAL_H_