[heap, offthread] Skip allocation steps for local spaces

Allocation observers are not thread-safe, so the allocation steps should
be invoked only on the main thread.

Bug: v8:10536
Change-Id: I90e809fa230d18cce135f4bea67b62ef616ca593
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2207189Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67891}

src/heap/paged-spaces.cc

@@ -404,7 +404,10 @@ void PagedSpace::FreeLinearAllocationArea() {
     }
   }
 
-  InlineAllocationStep(current_top, kNullAddress, kNullAddress, 0);
+  if (!is_local_space()) {
+    InlineAllocationStep(current_top, kNullAddress, kNullAddress, 0);
+  }
+
   SetTopAndLimit(kNullAddress, kNullAddress);
   DCHECK_GE(current_limit, current_top);