[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 f1c160f6049607849ff4f432f7d949492f7e5e14..5b5aca4d5f1d3a9ba372676150db9ba0871c2778 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 {
@@ -138,6 +140,34 @@ MUST_USE_RESULT Object* ConvertToUpper(Handle<String> s, Isolate* isolate);
 MUST_USE_RESULT Object* ConvertCase(Handle<String> s, bool is_upper,
                                     Isolate* isolate);
 
+// 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 = 100;
+  char timezone_name_[kMaxTimezoneChars];
+  char dst_timezone_name_[kMaxTimezoneChars];
+};
+
 }  // namespace internal
 }  // namespace v8