[gn] Enable stricter build flags

src/value-serializer.cc

 // 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.
 
 #include "src/value-serializer.h"
 
 #include <type_traits>
 
 #include "src/base/logging.h"
 #include "src/conversions.h"
@@ skipping 505 matching lines @@
   }
 
   WriteTag(SerializationTag::kEndJSObject);
   WriteVarint<uint32_t>(properties_written);
   return ThrowIfOutOfMemory();
 }
 
 Maybe<bool> ValueSerializer::WriteJSObjectSlow(Handle<JSObject> object) {
   WriteTag(SerializationTag::kBeginJSObject);
   Handle<FixedArray> keys;
-  uint32_t properties_written;
+  uint32_t properties_written = 0;
   if (!KeyAccumulator::GetKeys(object, KeyCollectionMode::kOwnOnly,
                                ENUMERABLE_STRINGS)
            .ToHandle(&keys) ||
       !WriteJSObjectPropertiesSlow(object, keys).To(&properties_written)) {
     return Nothing<bool>();
   }
   WriteTag(SerializationTag::kEndJSObject);
   WriteVarint<uint32_t>(properties_written);
   return ThrowIfOutOfMemory();
 }
@@ skipping 88 matching lines @@
     if (!WriteJSObjectPropertiesSlow(array, keys).To(&properties_written)) {
       return Nothing<bool>();
     }
     WriteTag(SerializationTag::kEndDenseJSArray);
     WriteVarint<uint32_t>(properties_written);
     WriteVarint<uint32_t>(length);
   } else {
     WriteTag(SerializationTag::kBeginSparseJSArray);
     WriteVarint<uint32_t>(length);
     Handle<FixedArray> keys;
-    uint32_t properties_written;
+    uint32_t properties_written = 0;
     if (!KeyAccumulator::GetKeys(array, KeyCollectionMode::kOwnOnly,
                                  ENUMERABLE_STRINGS)
              .ToHandle(&keys) ||
         !WriteJSObjectPropertiesSlow(array, keys).To(&properties_written)) {
       return Nothing<bool>();
     }
     WriteTag(SerializationTag::kEndSparseJSArray);
     WriteVarint<uint32_t>(properties_written);
     WriteVarint<uint32_t>(length);
   }
@@ skipping 1323 matching lines @@
   if (stack.size() != 1) {
     isolate_->Throw(*isolate_->factory()->NewError(
         MessageTemplate::kDataCloneDeserializationError));
     return MaybeHandle<Object>();
   }
   return scope.CloseAndEscape(stack[0]);
 }
 
 }  // namespace internal
 }  // namespace v8