Add pointer cache field to external string for access in generated code.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index cbf0cb316379a1966f9648671bad360c042d59db..8aa75804f61942169b49a23d24127748713ef6e7 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -2847,14 +2847,14 @@ MaybeObject* Heap::AllocateConsString(String* first, String* second) {
       // Copy first part.
       const char* src;
       if (first->IsExternalString()) {
-        src = ExternalAsciiString::cast(first)->resource()->data();
+        src = ExternalAsciiString::cast(first)->GetChars();
       } else {
         src = SeqAsciiString::cast(first)->GetChars();
       }
       for (int i = 0; i < first_length; i++) *dest++ = src[i];
       // Copy second part.
       if (second->IsExternalString()) {
-        src = ExternalAsciiString::cast(second)->resource()->data();
+        src = ExternalAsciiString::cast(second)->GetChars();
       } else {
         src = SeqAsciiString::cast(second)->GetChars();
       }