[offthread] Set LO space flags on merge

Make sure to set the right page flags on pages merged from off-thread
LO space have the right flags set -- in particular, the marking bit.

Bug: chromium:1011762
Change-Id: I595a3afb5e40d92b8d4c26c7671a43ea8e9e94d2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2169096
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67439}

src/heap/spaces.cc

@@ -4284,7 +4284,6 @@ void OldLargeObjectSpace::PromoteNewLargeObject(LargePage* page) {
   size_t object_size = static_cast<size_t>(page->GetObject().Size());
   static_cast<LargeObjectSpace*>(page->owner())->RemovePage(page, object_size);
   page->ClearFlag(MemoryChunk::FROM_PAGE);
-  page->SetOldGenerationPageFlags(heap()->incremental_marking()->IsMarking());
   AddPage(page, object_size);
 }
 
@@ -4295,6 +4294,7 @@ void LargeObjectSpace::AddPage(LargePage* page, size_t object_size) {
   page_count_++;
   memory_chunk_list_.PushBack(page);
   page->set_owner(this);
+  page->SetOldGenerationPageFlags(heap()->incremental_marking()->IsMarking());
 }
 
 void LargeObjectSpace::RemovePage(LargePage* page, size_t object_size) {