cppgc: Add CHECK for diagnosing Persistent issue

Bug: chromium:1253650
Change-Id: I289dae157408ab68d76fb168aadf7924c3a65c09
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3302798Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78102}

include/cppgc/persistent.h

@@ -189,6 +189,9 @@ class BasicPersistent final : public PersistentBase,
   // heterogeneous assignments between different Member and Persistent handles
   // based on their actual types.
   V8_CLANG_NO_SANITIZE("cfi-unrelated-cast") T* Get() const {
+    // TODO(chromium:1253650): Temporary CHECK to diagnose issues.
+    CPPGC_CHECK(IsValid() == !!GetNode());
+
     // The const_cast below removes the constness from PersistentBase storage.
     // The following static_cast re-adds any constness if specified through the
     // user-visible template parameter T.
@@ -196,6 +199,8 @@ class BasicPersistent final : public PersistentBase,
   }
 
   void Clear() {
+    // TODO(chromium:1253650): Temporary CHECK to diagnose issues.
+    CPPGC_CHECK(IsValid() == !!GetNode());
     // Simplified version of `Assign()` to allow calling without a complete type
     // `T`.
     if (IsValid()) {