Revert of Cleanup sign checking in safe math code

base/numerics/safe_conversions_impl.h

 // Copyright 2014 The Chromium 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 BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
 #define BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
 
 #include <limits.h>
 #include <stdint.h>
 
@@ skipping 272 matching lines @@
 static_assert(IntegerBitsPlusSign<intmax_t>::value == 64,
               "Max integer size not supported for this toolchain.");
 
 template <typename Integer, bool IsSigned = std::is_signed<Integer>::value>
 struct TwiceWiderInteger {
   using type =
       typename IntegerForDigitsAndSign<IntegerBitsPlusSign<Integer>::value * 2,
                                        IsSigned>::type;
 };
 
+template <typename Integer>
+struct PositionOfSignBit {
+  static const size_t value = IntegerBitsPlusSign<Integer>::value - 1;
+};
+
 enum ArithmeticPromotionCategory {
   LEFT_PROMOTION,  // Use the type of the left-hand argument.
   RIGHT_PROMOTION  // Use the type of the right-hand argument.
 };
 
 // Determines the type that can represent the largest positive value.
 template <typename Lhs,
           typename Rhs,
           ArithmeticPromotionCategory Promotion =
               (MaxExponent<Lhs>::value > MaxExponent<Rhs>::value)
@@ skipping 301 matching lines @@
                    static_cast<BigType>(static_cast<L>(lhs)),
                    static_cast<BigType>(static_cast<R>(rhs)))
              // Let the template functions figure it out for mixed types.
              : C<L, R>::Test(lhs, rhs);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_