[heap] Expand verification of layout changes to non-JSObject objects.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index cb9f46782de264d3272baf809edce8f88b68ac89..1015667dc71edfc77256526311af4f0effb01ebf 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -873,8 +873,11 @@ void IncrementalMarking::VisitObject(Map* map, HeapObject* obj, int size) {
   // 1. The object is a fixed array with the progress bar.
   // 2. The object is a JSObject that was colored black before
   //    unsafe layout change.
+  // 3. The object is a string that was colored black before
+  //    unsafe layout change.
   if (!ObjectMarking::GreyToBlack<kAtomicity>(obj, marking_state(obj))) {
-    DCHECK(IsFixedArrayWithProgressBar(obj) || obj->IsJSObject());
+    DCHECK(IsFixedArrayWithProgressBar(obj) || obj->IsJSObject() ||
+           obj->IsString());
   }
   DCHECK(ObjectMarking::IsBlack<kAtomicity>(obj, marking_state(obj)));
   WhiteToGreyAndPush(map);