Revert of Cleanup sign checking in safe math code

base/numerics/safe_conversions_impl.h

Index: base/numerics/safe_conversions_impl.h
diff --git a/base/numerics/safe_conversions_impl.h b/base/numerics/safe_conversions_impl.h
index 79dee83e85a00fcc66ff141a74f6b27269cb0d27..ef5adf0bf9ab49f9df9a5e5d80a2c3b08b9fe707 100644
--- a/base/numerics/safe_conversions_impl.h
+++ b/base/numerics/safe_conversions_impl.h
@@ -290,6 +290,11 @@
                                        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.