Reland "cppgc: Consistently treat sentinel pointer as live"

This is a reland of commit 60e9b503

Original change's description:
> cppgc: Consistently treat sentinel pointer as live
>
> Sentinel pointers would be treated as live by the GC (through
> `HandleWeak()` but would be treated as dead when checked explicitly
> through the `LivenessBroker` in e.g. custom callbacks.
>
> Treat sentinel pointers as live consistently across all callsites
> and weak types.
>
> Change-Id: I9a4c096ddac1a111df808f3683325b55e7597eea
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3782800
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Omer Katz <omerkatz@chromium.org>
> Reviewed-by: Anton Bikineev <bikineev@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#81916}

Change-Id: Ie2476345b9ea8406015a3b07bd6880c1159ede08
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3779913Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81975}

include/cppgc/liveness-broker.h

@@ -7,6 +7,7 @@
 
 #include "cppgc/heap.h"
 #include "cppgc/member.h"
+#include "cppgc/sentinel-pointer.h"
 #include "cppgc/trace-trait.h"
 #include "v8config.h"  // NOLINT(build/include_directory)
 
@@ -44,24 +45,24 @@ class V8_EXPORT LivenessBroker final {
  public:
   template <typename T>
   bool IsHeapObjectAlive(const T* object) const {
-    // nullptr objects are considered alive to allow weakness to be used from
+    // - nullptr objects are considered alive to allow weakness to be used from
     // stack while running into a conservative GC. Treating nullptr as dead
-    // would mean that e.g. custom collectins could not be strongified on stack.
-    return !object ||
+    // would mean that e.g. custom collections could not be strongified on
+    // stack.
+    // - Sentinel pointers are also preserved in weakness and not cleared.
+    return !object || object == kSentinelPointer ||
            IsHeapObjectAliveImpl(
                TraceTrait<T>::GetTraceDescriptor(object).base_object_payload);
   }
 
   template <typename T>
   bool IsHeapObjectAlive(const WeakMember<T>& weak_member) const {
-    return (weak_member != kSentinelPointer) &&
-           IsHeapObjectAlive<T>(weak_member.Get());
+    return IsHeapObjectAlive<T>(weak_member.Get());
   }
 
   template <typename T>
   bool IsHeapObjectAlive(const UntracedMember<T>& untraced_member) const {
-    return (untraced_member != kSentinelPointer) &&
-           IsHeapObjectAlive<T>(untraced_member.Get());
+    return IsHeapObjectAlive<T>(untraced_member.Get());
   }
 
  private:

include/cppgc/visitor.h

@@ -327,7 +327,6 @@ class V8_EXPORT Visitor {
     const PointerType* weak = static_cast<const PointerType*>(object);
     auto* raw_ptr = weak->GetFromGC();
     // Sentinel values are preserved for weak pointers.
-    if (raw_ptr == kSentinelPointer) return;
     if (!info.IsHeapObjectAlive(raw_ptr)) {
       weak->ClearFromGC();
     }