Changing Size to SizeOfObjects in GetHeapSpaceStatistics api.

This CL changes GetHeapSpaceStatistics api to return SizeOfObjects()
instead of Size(). SizeOfObjects is the required field and is
consistent with GetHeapStatistics api.

BUG=476013
LOG=N

Review URL: https://codereview.chromium.org/1095393005

Cr-Commit-Position: refs/heads/master@{#28080}

src/api.cc

@@ -6894,7 +6894,7 @@ bool Isolate::GetHeapSpaceStatistics(HeapSpaceStatistics* space_statistics,
 
   space_statistics->space_name_ = heap->GetSpaceName(static_cast<int>(index));
   space_statistics->space_size_ = space->CommittedMemory();
-  space_statistics->space_used_size_ = space->Size();
+  space_statistics->space_used_size_ = space->SizeOfObjects();
   space_statistics->space_available_size_ = space->Available();
   space_statistics->physical_space_size_ = space->CommittedPhysicalMemory();
   return true;