Provide more accurate results about used heap size via GetHeapStatistics.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 9da3346d7940cebbb49d60813c04d56a4466260f..3377ccd8062742e3410a8622581da25038108396 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -3272,7 +3272,7 @@ void v8::V8::GetHeapStatistics(HeapStatistics* heap_statistics) {
   heap_statistics->set_total_heap_size(i::Heap::CommittedMemory());
   heap_statistics->set_total_heap_size_executable(
       i::Heap::CommittedMemoryExecutable());
-  heap_statistics->set_used_heap_size(i::Heap::SizeOfObjects());
+  heap_statistics->set_used_heap_size(i::Heap::SizeOfObjectsSlow());
 }