Revert r24737 "Read object pointer atomically while updating slots"

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index f309eae9d1406150cd4dba059e238e0254d7c4ae..bcfa2cbfdd3e2eb001dfeb2e222063b848a568d8 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2926,8 +2926,7 @@ class PointersUpdatingVisitor : public ObjectVisitor {
   }
 
   static inline void UpdateSlot(Heap* heap, Object** slot) {
-    Object* obj = reinterpret_cast<Object*>(
-        base::NoBarrier_Load(reinterpret_cast<base::AtomicWord*>(slot)));
+    Object* obj = *slot;
 
     if (!obj->IsHeapObject()) return;
 
@@ -2938,10 +2937,7 @@ class PointersUpdatingVisitor : public ObjectVisitor {
       DCHECK(heap->InFromSpace(heap_obj) ||
              MarkCompactCollector::IsOnEvacuationCandidate(heap_obj));
       HeapObject* target = map_word.ToForwardingAddress();
-      base::NoBarrier_CompareAndSwap(
-          reinterpret_cast<base::AtomicWord*>(slot),
-          reinterpret_cast<base::AtomicWord>(obj),
-          reinterpret_cast<base::AtomicWord>(target));
+      *slot = target;
       DCHECK(!heap->InFromSpace(target) &&
              !MarkCompactCollector::IsOnEvacuationCandidate(target));
     }