Fix assertion failure caused by external strings.

src/extensions/externalize-string-extension.cc

Index: src/extensions/externalize-string-extension.cc
diff --git a/src/extensions/externalize-string-extension.cc b/src/extensions/externalize-string-extension.cc
index d372cf012577102eef6309065b6676984c5c18fb..adc5577d9e5adf30491b078271c1b7e6afe69697 100644
--- a/src/extensions/externalize-string-extension.cc
+++ b/src/extensions/externalize-string-extension.cc
@@ -107,7 +107,7 @@ void ExternalizeStringExtension::Externalize(
     SimpleAsciiStringResource* resource = new SimpleAsciiStringResource(
         reinterpret_cast<char*>(data), string->length());
     result = string->MakeExternal(resource);
-    if (result && !string->IsInternalizedString()) {
+    if (result) {
       i::Isolate* isolate = reinterpret_cast<i::Isolate*>(args.GetIsolate());
       isolate->heap()->external_string_table()->AddString(*string);
     }
@@ -118,7 +118,7 @@ void ExternalizeStringExtension::Externalize(
     SimpleTwoByteStringResource* resource = new SimpleTwoByteStringResource(
         data, string->length());
     result = string->MakeExternal(resource);
-    if (result && !string->IsInternalizedString()) {
+    if (result) {
       i::Isolate* isolate = reinterpret_cast<i::Isolate*>(args.GetIsolate());
       isolate->heap()->external_string_table()->AddString(*string);
     }