[heap] Introduce barrier when merging LocalSpace

Ensure that page header flags are properly initialized before concurrent markers can discover objects.

Bug: v8:10315
Change-Id: Ib9ce813dfd3fda75e4ee209a846828366222b110
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2198990Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67775}

src/heap/spaces.cc

@@ -1593,6 +1593,10 @@ void PagedSpace::MergeLocalSpace(LocalSpace* other) {
       p->MergeOldToNewRememberedSets();
     }
 
+    // Ensure that pages are initialized before objects on it are discovered by
+    // concurrent markers.
+    p->InitializationMemoryFence();
+
     // Relinking requires the category to be unlinked.
     other->RemovePage(p);
     AddPage(p);