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

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index 51c037c9b7e20085e226ea563de85137dfde4aa8..3105a819e44b1cfc281e7be5b195d3f2de8815b6 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -253,6 +253,9 @@ 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();