Make base/numerics abs match runtime libraries

This is the version all the runtime libraries seem to
use. So, I'm matching it on the assumption the
compiler may do something smarter if appropriate.

TBR=scottmg
NOTRY=true

Review-Url: https://codereview.chromium.org/2574443002
Cr-Commit-Position: refs/heads/master@{#437996}

base/numerics/safe_conversions_impl.h

@@ -54,7 +54,7 @@ constexpr T BinaryComplement(T x) {
 template <typename T>
 constexpr T SafeUnsignedAbsImpl(T value, T sign_mask) {
   static_assert(!std::is_signed<T>::value, "Types must be unsigned.");
-  return (value + sign_mask) ^ sign_mask;
+  return (value ^ sign_mask) - sign_mask;
 }
 
 template <typename T,