Clear recorded slots when making a string external.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 38deeb825d4b291c707d5e76c1f3cffbf4f1b94a..6ed5f68485b6236bdcba91c90b4c9f447534e88e 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -2093,6 +2093,7 @@ bool String::MakeExternal(v8::String::ExternalOneByteStringResource* resource) {
   int new_size = this->SizeFromMap(new_map);
   heap->CreateFillerObjectAt(this->address() + new_size, size - new_size,
                              ClearRecordedSlots::kNo);
+  heap->ClearRecordedSlotRange(this->address(), this->address() + new_size);
 
   // We are storing the new map using release store after creating a filler for
   // the left-over space to avoid races with the sweeper thread.