cppgc: Check AssignUnsafe use cases

Assert that the lock is help whenever AssignUnsafe is called.
LazyMutex::AssertHeld resolves to a DCHECK so this should not
regress production performance (other than the call itself
that might not be inlined).

Bug: chromium:1056170
Change-Id: Ic2005d180e6960c24dff7743aa3e0d5e57a63d80
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2716286
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73000}

include/cppgc/cross-thread-persistent.h

@@ -271,6 +271,7 @@ class BasicCrossThreadPersistent final : public PersistentBase,
   }
 
   void AssignUnsafe(T* ptr) {
+    PersistentRegionLock::AssertLocked();
     const void* old_value = GetValue();
     if (IsValid(old_value)) {
       PersistentRegion& region = this->GetPersistentRegion(old_value);

include/cppgc/internal/persistent-node.h

@@ -121,6 +121,8 @@ class V8_EXPORT PersistentRegionLock final {
  public:
   PersistentRegionLock();
   ~PersistentRegionLock();
+
+  static void AssertLocked();
 };
 
 }  // namespace internal

src/heap/cppgc/persistent-node.cc

@@ -90,5 +90,10 @@ PersistentRegionLock::~PersistentRegionLock() {
   g_process_mutex.Pointer()->Unlock();
 }
 
+// static
+void PersistentRegionLock::AssertLocked() {
+  return g_process_mutex.Pointer()->AssertHeld();
+}
+
 }  // namespace internal
 }  // namespace cppgc