ThinStrings: fix Factory::NewProperSubString

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 92d287b88b4e420ca3b0035290275ebddf261aec..d892d5b994ae2fb469b48f6aad9e0359d6ba3d0f 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -795,6 +795,10 @@ Handle<String> Factory::NewProperSubString(Handle<String> str,
     str = Handle<String>(slice->parent(), isolate());
     offset += slice->offset();
   }
+  if (str->IsThinString()) {
+    Handle<ThinString> thin = Handle<ThinString>::cast(str);
+    str = handle(thin->actual(), isolate());
+  }
 
   DCHECK(str->IsSeqString() || str->IsExternalString());
   Handle<Map> map = str->IsOneByteRepresentation()