[cleanup] Add DCHECK for implicit assumption.

Bug:v8:6325

Change-Id: I0c66d24994a4b6811c7aec80f4aa298ba6d209e4
Reviewed-on: https://chromium-review.googlesource.com/490366Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44986}

src/profiler/heap-snapshot-generator.cc

@@ -270,6 +270,7 @@ HeapEntry* HeapSnapshot::AddEntry(HeapEntry::Type type,
                                   size_t size,
                                   unsigned trace_node_id) {
   HeapEntry entry(this, type, name, id, size, trace_node_id);
+  DCHECK(sorted_entries_.is_empty());
   entries_.Add(entry);
   return &entries_.last();
 }