[Sync] Rework SharedValue<T> into Immutable<T>

chrome/browser/sync/util/shared_value.h

-// Copyright (c) 2011 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 CHROME_BROWSER_SYNC_UTIL_SHARED_VALUE_H_
-#define CHROME_BROWSER_SYNC_UTIL_SHARED_VALUE_H_
-#pragma once
-
-#include "base/memory/ref_counted.h"
-
-namespace browser_sync {
-
-template <class ValueType>
-struct HasSwapMemFnTraits {
-  static void Swap(ValueType* t1, ValueType* t2) {
-    t1->Swap(t2);
-  }
-};
-
-// A ref-counted thread-safe wrapper around an immutable value.
-template <class ValueType, class Traits>
-class SharedValue
-    : public base::RefCountedThreadSafe<SharedValue<ValueType, Traits> > {
- public:
-  SharedValue() {}
-  // Takes over the data in |value|, leaving |value| empty.
-  explicit SharedValue(ValueType* value) {
-    Traits::Swap(&value_, value);
-  }
-
-  const ValueType& Get() const {
-    return value_;
-  }
-
- private:
-  ~SharedValue() {}
-  friend class base::RefCountedThreadSafe<SharedValue<ValueType, Traits> >;
-
-  ValueType value_;
-};
-
-}  // namespace browser_sync
-
-#endif  // CHROME_BROWSER_SYNC_UTIL_SHARED_VALUE_H_