Replaced different means of zone pooling/reusing by one zone segment pool

src/zone/zone.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/zone/zone.h"
 
 #include <cstring>
 
 #include "src/v8.h"
 
@@ skipping 87 matching lines @@
 
     // Un-poison the segment content so we can re-use or zap it later.
     ASAN_UNPOISON_MEMORY_REGION(current->start(), current->capacity());
 
     segment_bytes_allocated_ -= size;
     allocator_->ReturnSegment(current);
     current = next;
   }
 
   position_ = limit_ = 0;
-
   allocation_size_ = 0;
-  // Update the head segment to be the kept segment (if any).
   segment_head_ = nullptr;
 }
 
 // Creates a new segment, sets it size, and pushes it to the front
 // of the segment chain. Returns the new segment.
 Segment* Zone::NewSegment(size_t size) {
   Segment* result = allocator_->GetSegment(size);
   segment_bytes_allocated_ += size;
   if (result != nullptr) {
     result->Initialize(segment_head_, size, this);
@@ skipping 53 matching lines @@
   // size bytes + header and alignment padding)
   DCHECK(reinterpret_cast<uintptr_t>(position_) >=
          reinterpret_cast<uintptr_t>(result));
   limit_ = segment->end();
   DCHECK(position_ <= limit_);
   return result;
 }
 
 }  // namespace internal
 }  // namespace v8