Adding parenthesis around condition to fix IE8, after 912a2360

https://github.com/mootools/mootools-core/commit/912a23600d64d9c8030a58c700ac6617e09475ca#commitcomment-5228993
remove feature check for Presto
Merge pull request #2557 from gonchuki/removePrestoComputedStyleCheck

remove feature check for Presto