Pretenure LayoutDescriptor

LayoutDescriptor is currently the only case where the Scavenger needs to
potentially follow an updated slot to iterate an object. This scenario
requires at least Acq/Rel semantics.

In order to use relaxed store/load for the slots we need to allocate it
pretenured.

Bug: chromium:738865
Change-Id: I353fa6f252b436918b60fff54ece5ce5c7783072
Reviewed-on: https://chromium-review.googlesource.com/590429Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46965}

src/layout-descriptor-inl.h

@@ -22,7 +22,7 @@ Handle<LayoutDescriptor> LayoutDescriptor::New(Isolate* isolate, int length) {
   }
   int backing_store_length = GetSlowModeBackingStoreLength(length);
   Handle<LayoutDescriptor> result = Handle<LayoutDescriptor>::cast(
-      isolate->factory()->NewByteArray(backing_store_length));
+      isolate->factory()->NewByteArray(backing_store_length, TENURED));
   memset(result->GetDataStartAddress(), 0, result->DataSize());
   return result;
 }