[date] Refactor TimezoneCache to be separate from the OS

src/base/platform/platform-posix.h

+// Copyright 2017 the V8 project 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 V8_BASE_PLATFORM_PLATFORM_POSIX_H_
+#define V8_BASE_PLATFORM_PLATFORM_POSIX_H_
+
+#include "src/base/timezone-cache.h"
+
+namespace v8 {
+namespace base {
+
+class PosixTimezoneCache : public TimezoneCache {
+ public:
+  const char* LocalTimezone(double time_ms) override;
+  double DaylightSavingsOffset(double time_ms) override;
+  double LocalTimeOffset() override;
+  void Clear() override {}
+  ~PosixTimezoneCache() override {}
+
+ protected:
+  static const int msPerSecond = 1000;
+};
+
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_PLATFORM_PLATFORM_POSIX_H_