sync: Gracefully handle very early shutdown

sync/internal_api/public/base/cancelation_observer.h

Index: sync/internal_api/public/base/cancelation_observer.h
diff --git a/sync/internal_api/public/base/cancelation_observer.h b/sync/internal_api/public/base/cancelation_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..9581004c7502a1485d4ae136bb893fbb935c68cc
--- /dev/null
+++ b/sync/internal_api/public/base/cancelation_observer.h
@@ -0,0 +1,25 @@
+// Copyright 2013 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 SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_OBSERVER_H_
+#define SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_OBSERVER_H_
+
+#include "sync/base/sync_export.h"
+
+namespace syncer {
+
+// Interface for classes that handle signals from the CancelationSignal.
+class SYNC_EXPORT_PRIVATE CancelationObserver {
+ public:
+  CancelationObserver();
+  virtual ~CancelationObserver() = 0;
+
+  // This may be called from a foreign thread while the CancelationSignal's lock
+  // is held.  The callee should avoid performing slow or blocking operations.
+  virtual void OnStopRequested() = 0;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_OBSERVER_H_