Added zone pool metrics to gc-trace.

src/zone/accounting-allocator.h

 // Copyright 2016 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_ZONE_ACCOUNTING_ALLOCATOR_H_
 #define V8_ZONE_ACCOUNTING_ALLOCATOR_H_
 
 #include "include/v8-platform.h"
 #include "src/base/atomic-utils.h"
 #include "src/base/atomicops.h"
@@ skipping 41 matching lines @@
   bool AddSegmentToPool(Segment* segment);
 
   // Empties the pool and puts all its contents onto the garbage stack.
   void ClearPool();
 
   Segment*
       unused_segments_heads_[1 + kMaxSegmentSizePower - kMinSegmentSizePower];
 
   size_t unused_segments_sizes[1 + kMaxSegmentSizePower - kMinSegmentSizePower];
 
-  size_t unused_segments_size_ = 0;
-
   base::Mutex unused_segments_mutex_;
 
   base::AtomicWord current_memory_usage_ = 0;
   base::AtomicWord max_memory_usage_ = 0;
+  base::AtomicWord current_pool_size_ = 0;
 
   base::AtomicValue<MemoryPressureLevel> memory_pressure_level_;
 
   DISALLOW_COPY_AND_ASSIGN(AccountingAllocator);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ZONE_ACCOUNTING_ALLOCATOR_H_