Introduce a PrototypeIterator template and use it all over the place

src/heap-snapshot-generator.cc

Index: src/heap-snapshot-generator.cc
diff --git a/src/heap-snapshot-generator.cc b/src/heap-snapshot-generator.cc
index be970eeeeb3c19bcdccb59f02eba2ecc6a187873..fbaff4c1d1e3662fc7cdb0cf0e5963626e3ec6b7 100644
--- a/src/heap-snapshot-generator.cc
+++ b/src/heap-snapshot-generator.cc
@@ -11,6 +11,7 @@
 #include "src/conversions.h"
 #include "src/debug.h"
 #include "src/heap-profiler.h"
+#include "src/prototype-iterator.h"
 #include "src/types.h"
 
 namespace v8 {
@@ -1160,7 +1161,7 @@ void V8HeapExplorer::ExtractJSObjectReferences(
   ExtractElementReferences(js_obj, entry);
   ExtractInternalReferences(js_obj, entry);
   SetPropertyReference(
-      obj, entry, heap_->proto_string(), js_obj->GetPrototype());
+      obj, entry, heap_->proto_string(), SAFE_GET_PROTOTYPE_FAST(js_obj));
   if (obj->IsJSFunction()) {
     JSFunction* js_fun = JSFunction::cast(js_obj);
     Object* proto_or_map = js_fun->prototype_or_initial_map();