[heap] Make slot set state and operations atomic.

src/base/atomic-utils.h

Index: src/base/atomic-utils.h
diff --git a/src/base/atomic-utils.h b/src/base/atomic-utils.h
index e19385dcb15742909517cb208f048d10dfee9ea7..31db603bf94345bb84ce9ef7b135c513f8f9da3a 100644
--- a/src/base/atomic-utils.h
+++ b/src/base/atomic-utils.h
@@ -72,6 +72,22 @@ class AtomicValue {
            cast_helper<T>::to_storage_type(old_value);
   }
 
+  V8_INLINE void SetBits(T bits, T mask) {
+    DCHECK_EQ(bits & ~mask, 0);
+    T old_value;
+    T new_value;
+    do {
+      old_value = Value();
+      new_value = (old_value & ~mask) | bits;
+    } while (!TrySetValue(old_value, new_value));
+  }
+
+  V8_INLINE void SetBit(int bit) {
+    SetBits(static_cast<T>(1) << bit, static_cast<T>(1) << bit);
+  }
+
+  V8_INLINE void ClearBit(int bit) { SetBits(0, 1 << bit); }
+
   V8_INLINE void SetValue(T new_value) {
     base::Release_Store(&value_, cast_helper<T>::to_storage_type(new_value));
   }