[proxies] Make Object.prototype.isPrototypeOf work with proxies.

src/prototype.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_PROTOTYPE_H_
 #define V8_PROTOTYPE_H_
 
 #include "src/isolate.h"
 #include "src/objects.h"
 
@@ skipping 88 matching lines @@
     } else {
       if (handle_.is_null()) {
         object_ = HeapObject::cast(object_)->map()->prototype();
       } else {
         handle_ =
             handle(HeapObject::cast(*handle_)->map()->prototype(), isolate_);
       }
     }
   }
 
+  // Returns false iff a call to JSProxy::GetPrototype throws.
+  // TODO(neis): This should probably replace Advance().
+  bool AdvanceFollowingProxies() {
+    DCHECK(!(handle_.is_null() && object_->IsJSProxy()));
+    if (!handle_.is_null() && handle_->IsJSProxy()) {
+      did_jump_to_prototype_chain_ = true;
+      MaybeHandle<Object> proto =
+          JSProxy::GetPrototype(Handle<JSProxy>::cast(handle_));
+      return proto.ToHandle(&handle_);
+    }
+    AdvanceIgnoringProxies();
+    return true;
+  }
+
   bool IsAtEnd(WhereToEnd where_to_end = END_AT_NULL) const {
     if (handle_.is_null()) {
       return object_->IsNull() ||
              (did_jump_to_prototype_chain_ &&
               where_to_end == END_AT_NON_HIDDEN &&
               !HeapObject::cast(object_)->map()->is_hidden_prototype());
     } else {
       return handle_->IsNull() ||
              (did_jump_to_prototype_chain_ &&
               where_to_end == END_AT_NON_HIDDEN &&
@@ skipping 19 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(PrototypeIterator);
 };
 
 
 }  // namespace internal
 
 }  // namespace v8
 
 #endif  // V8_PROTOTYPE_H_