Dramatically speed up detailed heap snapshot generation.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 3d88568882350628c8f4895c41c0265c9f502f8f..16ca173d2f0421bdf005a981181c2531926aac25 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1587,6 +1587,7 @@ class JSObject: public HeapObject {
   inline int GetHeaderSize();
 
   inline int GetInternalFieldCount();
+  inline int GetInternalFieldOffset(int index);
   inline Object* GetInternalField(int index);
   inline void SetInternalField(int index, Object* value);
 
@@ -1700,6 +1701,7 @@ class JSObject: public HeapObject {
       int unused_property_fields);
 
   // Access fast-case object properties at index.
+  inline int GetFastPropertyOffset(int index);
   inline Object* FastPropertyAt(int index);
   inline Object* FastPropertyAtPut(int index, Object* value);