[heap] Remove unnecessary Relaxed_Stores in Scavenger.

(Simplifying the code in preparation for in-place weak references.)

BUG=v8:7308

Change-Id: I8348e108ae12bcd5b218f2d987f34017a1eb5394
Reviewed-on: https://chromium-review.googlesource.com/928421
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51424}

src/heap/scavenger-inl.h

@@ -228,9 +228,8 @@ void Scavenger::ScavengeObject(HeapObject** p, HeapObject* object) {
   // If the first word is a forwarding address, the object has already been
   // copied.
   if (first_word.IsForwardingAddress()) {
-    HeapObject* dest = first_word.ToForwardingAddress();
-    DCHECK(object->GetIsolate()->heap()->InFromSpace(*p));
-    base::AsAtomicPointer::Relaxed_Store(p, dest);
+    DCHECK(heap()->InFromSpace(*p));
+    *p = first_word.ToForwardingAddress();
     return;
   }