Convince gcc that local variables in i::ValueDeserializer::ReadJSArrayBufferView are initialized.

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 1123 matching lines @@
   }
   Handle<JSArrayBuffer> array_buffer(
       JSArrayBuffer::cast(transfer_map->ValueAt(index)), isolate_);
   AddObjectWithID(id, array_buffer);
   return array_buffer;
 }
 
 MaybeHandle<JSArrayBufferView> ValueDeserializer::ReadJSArrayBufferView(
     Handle<JSArrayBuffer> buffer) {
   uint32_t buffer_byte_length = NumberToUint32(buffer->byte_length());
-  uint8_t tag;
-  uint32_t byte_offset;
-  uint32_t byte_length;
+  uint8_t tag = 0;
+  uint32_t byte_offset = 0;
+  uint32_t byte_length = 0;
   if (!ReadVarint<uint8_t>().To(&tag) ||
       !ReadVarint<uint32_t>().To(&byte_offset) ||
       !ReadVarint<uint32_t>().To(&byte_length) ||
       byte_offset > buffer_byte_length ||
       byte_length > buffer_byte_length - byte_offset) {
     return MaybeHandle<JSArrayBufferView>();
   }
   uint32_t id = next_id_++;
   ExternalArrayType external_array_type = kExternalInt8Array;
   unsigned element_size = 0;
@@ skipping 178 matching lines @@
   }
 #endif
   position_ = end_;
 
   if (stack.size() != 1) return MaybeHandle<Object>();
   return scope.CloseAndEscape(stack[0]);
 }
 
 }  // namespace internal
 }  // namespace v8