Revert of [Sync] Refactor ModelSafeWorker::DoWorkAndWaitUntilDone() to avoid code duplication.

components/sync/base/scoped_event_signal.h

Index: components/sync/base/scoped_event_signal.h
diff --git a/components/sync/base/scoped_event_signal.h b/components/sync/base/scoped_event_signal.h
new file mode 100644
index 0000000000000000000000000000000000000000..d8df1044afdb84bcfe9d7471d697dc5eb961ee60
--- /dev/null
+++ b/components/sync/base/scoped_event_signal.h
@@ -0,0 +1,40 @@
+// 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 {
+class WaitableEvent;
+}
+
+namespace syncer {
+
+// An object which signals a WaitableEvent when it is deleted. Used to wait for
+// a task to run or be abandoned.
+class ScopedEventSignal {
+ public:
+  // |event| will be signaled in the destructor.
+  explicit ScopedEventSignal(base::WaitableEvent* event);
+
+  // 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();
+
+ private:
+  base::WaitableEvent* event_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedEventSignal);
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_BASE_SCOPED_EVENT_SIGNAL_H_