Recommit introducing short external strings.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 8aa75804f61942169b49a23d24127748713ef6e7..02b4ca11bde91d816f8d6dfcb24bb5dcc9092a86 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -3997,6 +3997,15 @@ Map* Heap::SymbolMapForString(String* string) {
   if (map == external_string_with_ascii_data_map()) {
     return external_symbol_with_ascii_data_map();
   }
+  if (map == short_external_string_map()) {
+    return short_external_symbol_map();
+  }
+  if (map == short_external_ascii_string_map()) {
+    return short_external_ascii_symbol_map();
+  }
+  if (map == short_external_string_with_ascii_data_map()) {
+    return short_external_symbol_with_ascii_data_map();
+  }
 
   // No match found.
   return NULL;