[date] Refactor TimezoneCache to be separate from the OS

src/base/timezone-cache.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_TIMEZONE_CACHE_H_
+#define V8_BASE_TIMEZONE_CACHE_H_
+
+namespace v8 {
+namespace base {
+
+class TimezoneCache {
+ public:
+  // Short name of the local timezone (e.g., EST)
+  virtual const char* LocalTimezone(double time_ms) = 0;
+
+  // ES #sec-daylight-saving-time-adjustment
+  // Daylight Saving Time Adjustment
+  virtual double DaylightSavingsOffset(double time_ms) = 0;
+
+  // ES #sec-local-time-zone-adjustment
+  // Local Time Zone Adjustment
+  //
+  // TODO(littledan): Make more accurate with another parameter along the
+  // lines of this spec change:
+  // https://github.com/tc39/ecma262/pull/778
+  virtual double LocalTimeOffset() = 0;
+
+  // Called when the local timezone changes
+  virtual void Clear() = 0;
+
+  // Called when tearing down the isolate
+  virtual ~TimezoneCache() {}
+};
+
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_TIMEZONE_CACHE_H_