[date] Add a cache for timezone names to DateCache

src/date.h

 // Copyright 2012 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_DATE_H_
 #define V8_DATE_H_
 
 #include "src/allocation.h"
 #include "src/base/platform/platform.h"
 #include "src/base/timezone-cache.h"
@@ skipping 75 matching lines @@
       local_offset_ms_ = GetLocalOffsetFromOS();
     }
     return local_offset_ms_;
   }
 
 
   const char* LocalTimezone(int64_t time_ms) {
     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
   int TimezoneOffset(int64_t time_ms) {
     int64_t local_ms = ToLocal(time_ms);
     return static_cast<int>((time_ms - local_ms) / kMsPerMin);
   }
 
   // ECMA 262 - 15.9.1.9
   // LocalTime(t) = t + LocalTZA + DaylightSavingTA(t)
@@ skipping 162 matching lines @@
 
   int local_offset_ms_;
 
   // Year/Month/Day cache.
   bool ymd_valid_;
   int ymd_days_;
   int ymd_year_;
   int ymd_month_;
   int ymd_day_;
 
+  // Timezone name cache
+  const char* tz_name_;
+  const char* dst_tz_name_;
+
   base::TimezoneCache* tz_cache_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif