Make maps in monomorphic IC stubs weak.

src/objects-visiting-inl.h

Index: src/objects-visiting-inl.h
diff --git a/src/objects-visiting-inl.h b/src/objects-visiting-inl.h
index 31117bb945bb62991245ef676beac33314940167..ba31b54f605682be03063b82ebd64f387f45f0ea 100644
--- a/src/objects-visiting-inl.h
+++ b/src/objects-visiting-inl.h
@@ -312,7 +312,8 @@ void StaticMarkingVisitor<StaticVisitor>::VisitCodeTarget(
   if (FLAG_cleanup_code_caches_at_gc && target->is_inline_cache_stub()
       && (target->ic_state() == MEGAMORPHIC || target->ic_state() == GENERIC ||
           target->ic_state() == POLYMORPHIC || heap->flush_monomorphic_ics() ||
-          Serializer::enabled() || target->ic_age() != heap->global_ic_age())) {
+          Serializer::enabled() || target->ic_age() != heap->global_ic_age() ||
+          target->is_invalidated_weak_stub())) {
     IC::Clear(target->GetIsolate(), rinfo->pc(),
               rinfo->host()->constant_pool());
     target = Code::GetCodeFromTargetAddress(rinfo->target_address());
@@ -656,7 +657,7 @@ void StaticMarkingVisitor<StaticVisitor>::MarkMapContents(
   // pointer to it.
   StaticVisitor::VisitPointers(heap,
       HeapObject::RawField(map, Map::kPointerFieldsBeginOffset),
-      HeapObject::RawField(map, Map::kPointerFieldsEndOffset));
+      HeapObject::RawField(map, Map::kFirstWeakFieldOffset));
 }