Fix detection of root Blink directory in webkitdirs.pm

Perl script was looking for a directory to determine root Blink
directory that got deleted in:
https://chromiumcodereview.appspot.com/21847002

Updated script to just check for Tools/Scripts instead.

BUG=268250

Review URL: https://chromiumcodereview.appspot.com/21683005

git-svn-id: svn://svn.chromium.org/blink/trunk@155657 bbb929c8-8fbe-4397-9dbb-9b2b20218538