base/numerics: Add explicit cast to SafeUnsignedAbs

This was showing up as a -Wconversion error on gcc, when used via
saturated_cast<int16_t>(...).

Change-Id: Ie643478c1678387ddc4b0386998c77bd74d92700
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2008277Reviewed-by: Justin Schuh <jschuh@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734082}

base/numerics/safe_conversions_impl.h

@@ -80,8 +80,9 @@ template <typename T>
 constexpr typename std::make_unsigned<T>::type SafeUnsignedAbs(T value) {
   static_assert(std::is_integral<T>::value, "Type must be integral");
   using UnsignedT = typename std::make_unsigned<T>::type;
-  return IsValueNegative(value) ? 0 - static_cast<UnsignedT>(value)
-                                : static_cast<UnsignedT>(value);
+  return IsValueNegative(value)
+             ? static_cast<UnsignedT>(0u - static_cast<UnsignedT>(value))
+             : static_cast<UnsignedT>(value);
 }
 
 // This allows us to switch paths on known compile-time constants.