[date] Add a cache for timezone names to DateCache

src/date.h

Index: src/date.h
diff --git a/src/date.h b/src/date.h
index 1a3b65638ce951909429613a1f4fd46345f46a80..5de2b84408634ef7b223ec7af3f3376cf7b73a74 100644
--- a/src/date.h
+++ b/src/date.h
@@ -93,7 +93,12 @@ class DateCache {
     if (time_ms < 0 || time_ms > kMaxEpochTimeInMs) {
       time_ms = EquivalentTime(time_ms);
     }
-    return tz_cache_->LocalTimezone(static_cast<double>(time_ms));
+    bool is_dst = DaylightSavingsOffsetInMs(time_ms) != 0;
+    const char** name = is_dst ? &dst_tz_name_ : &tz_name_;
+    if (*name == nullptr) {
+      *name = tz_cache_->LocalTimezone(static_cast<double>(time_ms));
+    }
+    return *name;
   }
 
   // ECMA 262 - 15.9.5.26
@@ -276,6 +281,10 @@ class DateCache {
   int ymd_month_;
   int ymd_day_;
 
+  // Timezone name cache
+  const char* tz_name_;
+  const char* dst_tz_name_;
+
   base::TimezoneCache* tz_cache_;
 };