Introduce a PrototypeIterator template and use it all over the place

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index a2f63202b3296d5a0f30538b4818d908c164036e..9224332324707ff6c2cd734a5d4c167bd1d700f2 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -6757,9 +6757,10 @@ HInstruction* HOptimizedGraphBuilder::BuildMonomorphicElementAccess(
     // changes could allow callbacks on elements in the chain that
     // aren't compatible with monomorphic keyed stores.
     Handle<JSObject> prototype(JSObject::cast(map->prototype()));
-    JSObject* holder = JSObject::cast(map->prototype());
-    while (!holder->GetPrototype()->IsNull()) {
-      holder = JSObject::cast(holder->GetPrototype());
+    JSObject* holder = NULL;
+    for (PrototypeIterator<STORE_AS_POINTER, MAP_BASED_WALK, END_AT_NULL_VALUE>
+             iter(*prototype); !iter.IsAtEnd(); iter.Advance()) {
+      holder = JSObject::cast(iter.GetCurrent());
     }
 
     BuildCheckPrototypeMaps(prototype,
@@ -7237,7 +7238,7 @@ HInstruction* HGraphBuilder::BuildCheckPrototypeMaps(Handle<JSObject> prototype,
                                                      Handle<JSObject> holder) {
   while (holder.is_null() || !prototype.is_identical_to(holder)) {
     BuildConstantMapCheck(prototype);
-    Object* next_prototype = prototype->GetPrototype();
+    Object* next_prototype = SAFE_GET_PROTOTYPE_FAST(*prototype);
     if (next_prototype->IsNull()) return NULL;
     CHECK(next_prototype->IsJSObject());
     prototype = handle(JSObject::cast(next_prototype));