Revert of Remove bad comment in HasSignBit

base/numerics/safe_math_impl.h

Index: base/numerics/safe_math_impl.h
diff --git a/base/numerics/safe_math_impl.h b/base/numerics/safe_math_impl.h
index b3f7f82445d2822aa88d434729f85d2962b2b263..f648d1642074f9071c66891e9bd2f4e8f7a7afb8 100644
--- a/base/numerics/safe_math_impl.h
+++ b/base/numerics/safe_math_impl.h
@@ -46,6 +46,7 @@
 
 template <typename T>
 constexpr bool HasSignBit(T x) {
+  // Cast to unsigned since right shift on signed is undefined.
   return static_cast<typename std::make_signed<T>::type>(x) <
          static_cast<typename std::make_signed<T>::type>(0);
 }