Verify a bit more about external strings

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 582038cca64ea6cd8e8ae83ced0b43ec80968d0a..f5d3cfaef495548eac08ce1c698cd460a7f6b1d4 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9287,6 +9287,8 @@ class ExternalString: public String {
  public:
   DECLARE_CAST(ExternalString)
 
+  inline const void* resource_data() const;
+
   // Layout description.
   static const int kResourceOffset = POINTER_SIZE_ALIGN(String::kSize);
   static const int kShortSize = kResourceOffset + kPointerSize;
@@ -9315,7 +9317,7 @@ class ExternalOneByteString : public ExternalString {
   typedef v8::String::ExternalOneByteStringResource Resource;
 
   // The underlying resource.
-  inline const Resource* resource();
+  inline const Resource* resource() const;
   inline void set_resource(const Resource* buffer);
 
   // Update the pointer cache to the external character array.
@@ -9351,7 +9353,7 @@ class ExternalTwoByteString: public ExternalString {
   typedef v8::String::ExternalStringResource Resource;
 
   // The underlying string resource.
-  inline const Resource* resource();
+  inline const Resource* resource() const;
   inline void set_resource(const Resource* buffer);
 
   // Update the pointer cache to the external character array.