DevTools: Fix find next shortcut on mac and remove it on linux.

This is a fix for a regression introduced in r161635

BUG=353221
R=pfeldman, lushnikov

Review URL: https://codereview.chromium.org/203183002

git-svn-id: svn://svn.chromium.org/blink/trunk@169458 bbb929c8-8fbe-4397-9dbb-9b2b20218538