cppgc: Add DCHECK to CppHeap::AdvanceTracing

Bug: chromium:1056170
Change-Id: I14fc220f3184b3472f60d54e3a2d6e554869b945
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3247191
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77569}

src/heap/cppgc-js/cpp-heap.cc

@@ -460,6 +460,7 @@ bool CppHeap::AdvanceTracing(double deadline_in_ms) {
       in_atomic_pause_ ? v8::base::TimeDelta::Max()
                        : v8::base::TimeDelta::FromMillisecondsD(deadline_in_ms);
   const size_t marked_bytes_limit = in_atomic_pause_ ? SIZE_MAX : 0;
+  DCHECK_NOT_NULL(marker_);
   // TODO(chromium:1056170): Replace when unified heap transitions to
   // bytes-based deadline.
   marking_done_ =