Introduce a PrototypeIterator template and use it all over the place

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 5240a70c205357020f940dedfe6ca71f71209f83..69b4b1573afe9c92c46685893375cf0c7314117b 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1505,7 +1505,10 @@ class Object {
       Handle<Object> receiver,
       uint32_t index);
 
-  // Return the object's prototype (might be Heap::null_value()).
+  // Return the object's prototype (might be Heap::null_value()). Please use a
+  // PrototypeIterator instead.
+  //
+  // TODO(jochen): Make these private.
   Object* GetPrototype(Isolate* isolate);
   static Handle<Object> GetPrototype(Isolate* isolate, Handle<Object> object);
 
@@ -1968,7 +1971,10 @@ class JSReceiver: public HeapObject {
       Handle<JSReceiver> object,
       uint32_t index);
 
-  // Return the object's prototype (might be Heap::null_value()).
+  // Return the object's prototype (might be Heap::null_value()). Use a
+  // PrototypeIterator instead.
+  //
+  // TODO(jochen): Make this method private.
   inline Object* GetPrototype();
 
   // Return the constructor function (may be Heap::null_value()).