[heap] Make object field write atomic if concurrent marking is enabled.

This patch also makes concurrent marking visitor loads atomic.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2872443003
Cr-Commit-Position: refs/heads/master@{#45179}

src/heap/concurrent-marking.cc

@@ -37,8 +37,10 @@ class ConcurrentMarkingVisitor final
 
   void VisitPointers(HeapObject* host, Object** start, Object** end) override {
     for (Object** p = start; p < end; p++) {
-      if (!(*p)->IsHeapObject()) continue;
-      MarkObject(HeapObject::cast(*p));
+      Object* object = reinterpret_cast<Object*>(
+          base::NoBarrier_Load(reinterpret_cast<const base::AtomicWord*>(p)));
+      if (!object->IsHeapObject()) continue;
+      MarkObject(HeapObject::cast(object));
     }
   }
 

src/objects-inl.h

@@ -1303,8 +1303,15 @@ bool JSObject::PrototypeHasNoElements(Isolate* isolate, JSObject* object) {
   reinterpret_cast<Object*>(base::NoBarrier_Load( \
       reinterpret_cast<const base::AtomicWord*>(FIELD_ADDR_CONST(p, offset))))
 
+#if V8_CONCURRENT_MARKING
+#define WRITE_FIELD(p, offset, value)                             \
+  base::NoBarrier_Store(                                          \
+      reinterpret_cast<base::AtomicWord*>(FIELD_ADDR(p, offset)), \
+      reinterpret_cast<base::AtomicWord>(value));
+#else
 #define WRITE_FIELD(p, offset, value) \
   (*reinterpret_cast<Object**>(FIELD_ADDR(p, offset)) = value)
+#endif
 
 #define RELEASE_WRITE_FIELD(p, offset, value)                     \
   base::Release_Store(                                            \