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..72b184a568ee83672162094fce0364f26ad6534a 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -2003,6 +2003,7 @@ bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
   Heap* heap = GetHeap();
   bool is_one_byte = this->IsOneByteRepresentation();
   bool is_internalized = this->IsInternalizedString();
+  bool has_pointers = this->IsConsString() || this->IsSlicedString();
 
   // Morph the string to an external string by replacing the map and
   // reinitializing the fields.  This won't work if the space the existing
@@ -2031,6 +2032,9 @@ bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
   int new_size = this->SizeFromMap(new_map);
   heap->CreateFillerObjectAt(this->address() + new_size, size - new_size,
                              ClearRecordedSlots::kNo);
+  if (has_pointers) {
+    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.
@@ -2071,6 +2075,7 @@ bool String::MakeExternal(v8::String::ExternalOneByteStringResource* resource) {
   if (size < ExternalString::kShortSize) return false;
   Heap* heap = GetHeap();
   bool is_internalized = this->IsInternalizedString();
+  bool has_pointers = this->IsConsString() || this->IsSlicedString();
 
   // Morph the string to an external string by replacing the map and
   // reinitializing the fields.  This won't work if the space the existing
@@ -2093,6 +2098,9 @@ 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);
+  if (has_pointers) {
+    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.