heap: Fix TracedReference::IsEmptyThreadSafe

* GetThreadSlot was missing const (Blink CL failed to build).
* Aligning IsEmptyThreadSafe implementation with IsEmpty.

Bug: chromium:1108537
Change-Id: I9c07e9a0e7f029cf3d70b4d2507900b600d064cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2330027
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69157}

include/v8.h

@@ -916,8 +916,8 @@ class TracedReferenceBase {
   /**
    * Get this reference in a thread-safe way
    */
-  T* GetSlotThreadSafe() {
-    return reinterpret_cast<std::atomic<T*>*>(&val_)->load(
+  const T* GetSlotThreadSafe() const {
+    return reinterpret_cast<std::atomic<const T*>*>(&val_)->load(
         std::memory_order_relaxed);
   }
 
@@ -1178,7 +1178,9 @@ class TracedReference : public TracedReferenceBase<T> {
    * Returns true if this TracedReference is empty, i.e., has not been
    * assigned an object. This version of IsEmpty is thread-safe.
    */
-  bool IsEmptyThreadSafe() const { return this->GetSlotThreadSafe(); }
+  bool IsEmptyThreadSafe() const {
+    return this->GetSlotThreadSafe() == nullptr;
+  }
 };
 
  /**