[heap] Fix new space size computation in gc tracer.

src/heap/gc-tracer.cc

 // Copyright 2014 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/heap/gc-tracer.h"
 
 #include "src/counters.h"
 #include "src/heap/heap-inl.h"
 #include "src/isolate.h"
 
@@ skipping 156 matching lines @@
         current_ = Event(Event::MARK_COMPACTOR, gc_reason, collector_reason);
       }
       break;
   }
 
   current_.reduce_memory = heap_->ShouldReduceMemory();
   current_.start_time = start_time;
   current_.start_object_size = heap_->SizeOfObjects();
   current_.start_memory_size = heap_->memory_allocator()->Size();
   current_.start_holes_size = CountTotalHolesSize(heap_);
-  current_.new_space_object_size =
-      heap_->new_space()->top() - heap_->new_space()->bottom();
+  current_.new_space_object_size = heap_->new_space()->Size();
 
   current_.incremental_marking_bytes = 0;
   current_.incremental_marking_duration = 0;
 
   for (int i = 0; i < Scope::NUMBER_OF_SCOPES; i++) {
     current_.scopes[i] = 0;
   }
 
   size_t committed_memory = heap_->CommittedMemory() / KB;
   size_t used_memory = current_.start_object_size / KB;
@@ skipping 637 matching lines @@
 }
 
 void GCTracer::ResetSurvivalEvents() { recorded_survival_ratios_.Reset(); }
 
 void GCTracer::NotifyIncrementalMarkingStart() {
   incremental_marking_start_time_ = heap_->MonotonicallyIncreasingTimeInMs();
 }
 
 }  // namespace internal
 }  // namespace v8