[gn] Enable stricter build flags

src/lookup.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_LOOKUP_H_
 #define V8_LOOKUP_H_
 
 #include "src/factory.h"
 #include "src/globals.h"
 #include "src/isolate.h"
@@ skipping 51 matching lines @@
                  Configuration configuration = DEFAULT)
       : configuration_(ComputeConfiguration(configuration, name)),
         interceptor_state_(InterceptorState::kUninitialized),
         property_details_(PropertyDetails::Empty()),
         isolate_(isolate),
         name_(isolate_->factory()->InternalizeName(name)),
         receiver_(receiver),
         initial_holder_(holder),
         // kMaxUInt32 isn't a valid index.
         index_(kMaxUInt32),
-        number_(DescriptorArray::kNotFound) {
+        number_(static_cast<uint32_t>(DescriptorArray::kNotFound)) {
 #ifdef DEBUG
     uint32_t index;  // Assert that the name is not an array index.
     DCHECK(!name->AsArrayIndex(&index));
 #endif  // DEBUG
     Start<false>();
   }
 
   LookupIterator(Isolate* isolate, Handle<Object> receiver, uint32_t index,
                  Configuration configuration = DEFAULT)
       : LookupIterator(isolate, receiver, index,
                        GetRoot(isolate, receiver, index), configuration) {}
 
   LookupIterator(Isolate* isolate, Handle<Object> receiver, uint32_t index,
                  Handle<JSReceiver> holder,
                  Configuration configuration = DEFAULT)
       : configuration_(configuration),
         interceptor_state_(InterceptorState::kUninitialized),
         property_details_(PropertyDetails::Empty()),
         isolate_(isolate),
         receiver_(receiver),
         initial_holder_(holder),
         index_(index),
-        number_(DescriptorArray::kNotFound) {
+        number_(static_cast<uint32_t>(DescriptorArray::kNotFound)) {
     // kMaxUInt32 isn't a valid index.
     DCHECK_NE(kMaxUInt32, index_);
     Start<true>();
   }
 
   static LookupIterator PropertyOrElement(
       Isolate* isolate, Handle<Object> receiver, Handle<Name> name,
       Configuration configuration = DEFAULT) {
     uint32_t index;
     if (name->AsArrayIndex(&index)) {
@@ skipping 260 matching lines @@
   const Handle<JSReceiver> initial_holder_;
   const uint32_t index_;
   uint32_t number_;
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_LOOKUP_H_