[date] Add a cache for timezone names to DateCache

src/date.cc

 // 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.
 
 #include "src/date.h"
 
 #include "src/objects.h"
 #include "src/objects-inl.h"
 
 namespace v8 {
@@ skipping 21 matching lines @@
   DCHECK(stamp_ != Smi::FromInt(kInvalidStamp));
   for (int i = 0; i < kDSTSize; ++i) {
     ClearSegment(&dst_[i]);
   }
   dst_usage_counter_ = 0;
   before_ = &dst_[0];
   after_ = &dst_[1];
   local_offset_ms_ = kInvalidLocalOffsetInMs;
   ymd_valid_ = false;
   tz_cache_->Clear();
+  tz_name_ = nullptr;
+  dst_tz_name_ = nullptr;
 }
 
 
 void DateCache::ClearSegment(DST* segment) {
   segment->start_sec = kMaxEpochTimeInSec;
   segment->end_sec = -kMaxEpochTimeInSec;
   segment->offset_ms = 0;
   segment->last_used = 0;
 }
 
@@ skipping 314 matching lines @@
     if (result == NULL || result->last_used > dst_[i].last_used) {
       result = &dst_[i];
     }
   }
   ClearSegment(result);
   return result;
 }
 
 }  // namespace internal
 }  // namespace v8