[gn] Enable stricter build flags

src/ast/context-slot-cache.h

 // Copyright 2016 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_CONTEXT_SLOT_CACHE_H_
 #define V8_AST_CONTEXT_SLOT_CACHE_H_
 
 #include "src/allocation.h"
 #include "src/ast/modules.h"
 
@@ skipping 20 matching lines @@
   // Clear the cache.
   void Clear();
 
   static const int kNotFound = -2;
 
  private:
   ContextSlotCache() {
     for (int i = 0; i < kLength; ++i) {
       keys_[i].data = NULL;
       keys_[i].name = NULL;
-      values_[i] = kNotFound;
+      values_[i] = static_cast<uint32_t>(kNotFound);
     }
   }
 
   inline static int Hash(Object* data, String* name);
 
 #ifdef DEBUG
   void ValidateEntry(Handle<Object> data, Handle<String> name,
                      VariableMode mode, InitializationFlag init_flag,
                      MaybeAssignedFlag maybe_assigned_flag, int slot_index);
 #endif
@@ skipping 51 matching lines @@
   uint32_t values_[kLength];
 
   friend class Isolate;
   DISALLOW_COPY_AND_ASSIGN(ContextSlotCache);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_CONTEXT_SLOT_CACHE_H_