uint128_struct: address misunderstood review feedback

util/numeric/int128.h

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
 #ifndef CRASHPAD_UTIL_NUMERIC_INT128_H_
 #define CRASHPAD_UTIL_NUMERIC_INT128_H_
 
 #include <stdint.h>
 
+#include "base/basictypes.h"
 #include "build/build_config.h"
 
 namespace crashpad {
 
 //! \brief Stores a 128-bit quantity.
 //!
 //! This structure is organized so that 128-bit quantities are laid out in
 //! memory according to the system’s natural byte order. If a system offers a
 //! native 128-bit type, it should be possible to bit_cast<> between that type
 //! and this one.
 //!
 //! This structure is designed to have the same layout, although not the same
 //! field names, as the Windows SDK’s `M128A` type from `<winnt.h>`. It is
 //! provided here instead of in `compat` because it is useful outside of the
 //! scope of data structures defined by the Windows SDK.
 struct uint128_struct {
 #if defined(ARCH_CPU_LITTLE_ENDIAN) || DOXYGEN
   //! \brief The low 64 bits of the 128-bit quantity.
   uint64_t lo;
 
   //! \brief The high 64 bits of the 128-bit quantity.
   uint64_t hi;
 #else
   uint64_t hi;
   uint64_t lo;
 #endif
 };
 
+COMPILE_ASSERT(sizeof(uint128_struct) == 16, uint128_must_be_16_bytes);
+
 }  // namespace crashpad
 
 #endif  // CRASHPAD_UTIL_NUMERIC_INT128_H_