Simplify saturated integer add/sub
https://bugs.webkit.org/show_bug.cgi?id=139854

Address Darin's post-commit feedback to fix API tests.

* wtf/SaturatedArithmetic.h: (saturatedSubtraction):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177736 268f45cc-cd09-0410-ab3c-d52691b4dbfc