[gn] Enable stricter build flags

src/ast/modules.h

 // Copyright 2012 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_AST_MODULES_H_
 #define V8_AST_MODULES_H_
 
 #include "src/parsing/scanner.h"  // Only for Scanner::Location.
 #include "src/zone/zone-containers.h"
 
@@ skipping 196 matching lines @@
   // (The import entry is never deleted.)
   void MakeIndirectExportsExplicit(Zone* zone);
 
   // Assign a cell_index of -1,-2,... to regular imports.
   // Assign a cell_index of +1,+2,... to regular (local) exports.
   // Assign a cell_index of 0 to anything else.
   void AssignCellIndices();
 
   int AddModuleRequest(const AstRawString* specifier) {
     DCHECK_NOT_NULL(specifier);
+    int module_requests_count = static_cast<int>(module_requests_.size());
     auto it = module_requests_
-                  .insert(std::make_pair(specifier, module_requests_.size()))
+                  .insert(std::make_pair(specifier, module_requests_count))
                   .first;
     return it->second;
   }
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_MODULES_H_