[proxies] do not leak private symbols to proxy traps

src/lookup.cc

 // 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.
 
 #include "src/lookup.h"
 
 #include "src/bootstrapper.h"
 #include "src/deoptimizer.h"
 #include "src/elements.h"
 #include "src/isolate-inl.h"
@@ skipping 591 matching lines @@
 
 LookupIterator::State LookupIterator::LookupInHolder(Map* const map,
                                                      JSReceiver* const holder) {
   STATIC_ASSERT(INTERCEPTOR == BEFORE_PROPERTY);
   DisallowHeapAllocation no_gc;
   if (interceptor_state_ == InterceptorState::kProcessNonMasking) {
     return LookupNonMaskingInterceptorInHolder(map, holder);
   }
   switch (state_) {
     case NOT_FOUND:
-      if (map->IsJSProxyMap()) return JSPROXY;
+      if (map->IsJSProxyMap()) {
+        if (!name_.is_null() && name_->IsPrivate()) return NOT_FOUND;
+        return JSPROXY;
+      }
       if (map->is_access_check_needed() &&
           (IsElement() || !isolate_->IsInternallyUsedPropertyName(name_))) {
         return ACCESS_CHECK;
       }
     // Fall through.
     case ACCESS_CHECK:
       if (exotic_index_state_ != ExoticIndexState::kNotExotic &&
           holder->IsJSTypedArray() && IsIntegerIndexedExotic(holder)) {
         return INTEGER_INDEXED_EXOTIC;
       }
@@ skipping 76 matching lines @@
     // Fall through.
     default:
       return NOT_FOUND;
   }
   UNREACHABLE();
   return state_;
 }
 
 }  // namespace internal
 }  // namespace v8