Move assertion about stopped marking up (again).

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index b3b2df9148fde8a0719bd91d3f214f1bd71d464e..a7c7b34eaea41b8e3eb9cc5a774537c0946249ac 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -651,9 +651,6 @@ bool Heap::CollectGarbage(AllocationSpace space,
           PerformGarbageCollection(collector, &tracer);
     }
 
-    ASSERT(collector == SCAVENGER || incremental_marking()->IsStopped());
-
-    // This can do debug callbacks and restart incremental marking.
     GarbageCollectionEpilogue();
   }
 
@@ -960,6 +957,10 @@ bool Heap::PerformGarbageCollection(GarbageCollector collector,
 
   isolate_->counters()->objs_since_last_young()->Set(0);
 
+  // Callbacks that fire after this point might trigger nested GCs and
+  // restart incremental marking, the assertion can't be moved down.
+  ASSERT(collector == SCAVENGER || incremental_marking()->IsStopped());
+
   gc_post_processing_depth_++;
   { DisableAssertNoAllocation allow_allocation;
     GCTracer::Scope scope(tracer, GCTracer::Scope::EXTERNAL);