[heap] Use SizeOfObjects() instead of Size() for accounting new_lo_space

The Size() includes the page header.

Bug: chromium:852420
Change-Id: I1f2da2309ff38f2222c82b5758937bdaf5036f12
Reviewed-on: https://chromium-review.googlesource.com/c/1458242Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59438}

src/heap/gc-tracer.cc

@@ -247,7 +247,7 @@ void GCTracer::Start(GarbageCollector collector,
   current_.start_memory_size = heap_->memory_allocator()->Size();
   current_.start_holes_size = CountTotalHolesSize(heap_);
   current_.young_object_size =
-      heap_->new_space()->Size() + heap_->new_lo_space()->Size();
+      heap_->new_space()->Size() + heap_->new_lo_space()->SizeOfObjects();
 
   current_.incremental_marking_bytes = 0;
   current_.incremental_marking_duration = 0;

src/heap/heap.cc

@@ -1687,7 +1687,7 @@ bool Heap::PerformGarbageCollection(
   EnsureFromSpaceIsCommitted();
 
   size_t start_young_generation_size =
-      Heap::new_space()->Size() + new_lo_space()->Size();
+      Heap::new_space()->Size() + new_lo_space()->SizeOfObjects();
 
   {
     Heap::SkipStoreBufferScope skip_store_buffer_scope(store_buffer_);