Wait for sweeper threads when verifying the heap.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index a209d25c796d2212d0aef9bde8b2b39270bc9220..3b36ef548f2c0e704639e47971d4ce95314f037b 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -4524,6 +4524,11 @@ void Heap::Verify() {
 
   store_buffer()->Verify();
 
+  if (mark_compact_collector()->sweeping_in_progress()) {
+    // We have to wait here for the sweeper threads to have an iterable heap.
+    mark_compact_collector()->EnsureSweepingCompleted();
+  }
+
   VerifyPointersVisitor visitor;
   IterateRoots(&visitor, VISIT_ONLY_STRONG);