Correcting a bogus assert outdated since r9295.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index bb24a2f8544ef73e069031be3bd38c0ed6315855..b8f3bd54228854508165a29a6240e6969e82abbf 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2336,7 +2336,7 @@ String* SlicedString::parent() {
 
 
 void SlicedString::set_parent(String* parent) {
-  ASSERT(parent->IsSeqString());
+  ASSERT(parent->IsSeqString() || parent->IsExternalString());
   WRITE_FIELD(this, kParentOffset, parent);
 }