[date] Add ICU backend for timezone info behind a flag

src/i18n.h

Index: src/i18n.h
diff --git a/src/i18n.h b/src/i18n.h
index f89d005b00c844309d925768f231c0bde94ca514..69a612120ce088f3ee60331b3b028e0f67bd2828 100644
--- a/src/i18n.h
+++ b/src/i18n.h
@@ -6,6 +6,7 @@
 #ifndef V8_I18N_H_
 #define V8_I18N_H_
 
+#include "src/base/timezone-cache.h"
 #include "src/objects.h"
 #include "unicode/uversion.h"
 
@@ -14,6 +15,7 @@ class BreakIterator;
 class Collator;
 class DecimalFormat;
 class SimpleDateFormat;
+class TimeZone;
 }
 
 namespace v8 {
@@ -128,6 +130,54 @@ class V8BreakIterator {
   V8BreakIterator();
 };
 
+// ICUTimezoneCache calls out to ICU for TimezoneCache
+// functionality in a straightforward way.
+class ICUTimezoneCache : public base::TimezoneCache {
+ public:
+  ICUTimezoneCache();
+
+  ~ICUTimezoneCache() override;
+
+  const char* LocalTimezone(double time_ms) override;
+
+  double DaylightSavingsOffset(double time_ms) override;
+
+  double LocalTimeOffset() override;
+
+  void Clear() override;
+
+ private:
+  icu::TimeZone* GetTimeZone();
+
+  bool GetOffsets(double time_ms, int32_t* raw_offset, int32_t* dst_offset);
+
+  icu::TimeZone* timezone_;
+
+  static const int32_t kMaxTimezoneChars = 20;
+  char timezone_name_[kMaxTimezoneChars];
+  char dst_timezone_name_[kMaxTimezoneChars];
+};
+
+// ICUOSTimezoneCache defers to the OS TimezoneCache
+// for LocalTimezone if ICU provides a timezone name
+// which starts with GMT. Otherwise, it uses ICU for
+// all operaitons. The issue here is that CLDR declines
+// to specify short names for many time zones, when the
+// OS often does know an appropriate short name.
+class ICUOSTimezoneCache : public ICUTimezoneCache {
+ public:
+  ICUOSTimezoneCache();
+
+  ~ICUOSTimezoneCache() override;
+
+  const char* LocalTimezone(double time_ms) override;
+
+  void Clear() override;
+
+ private:
+  TimezoneCache* os_tz_cache_;
+};
+
 }  // namespace internal
 }  // namespace v8