Introduce short external strings without pointer cache.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index 396f6789284e1cfa6d56d03b0e3581da6e55a760..8977cdb460213806429a6937fa2ac883be5c77f6 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -255,16 +255,11 @@ void Heap::FinalizeExternalString(String* string) {
           ExternalString::kResourceOffset -
           kHeapObjectTag);
 
-  // Clear pointer cache.
-  ExternalString::cast(string)->clear_data_cache();
-
   // Dispose of the C++ object if it has not already been disposed.
   if (*resource_addr != NULL) {
     (*resource_addr)->Dispose();
+    *resource_addr = NULL;
   }
-
-  // Clear the resource pointer in the string.
-  *resource_addr = NULL;
 }