Use double dashes for bench options to be consistent.

Breaks the bots, since the arguments being passed to bench have changed, but there is a change to fix it at https://codereview.appspot.com/6488114/

Review URL: https://codereview.appspot.com/6500108

git-svn-id: http://skia.googlecode.com/svn/trunk@5513 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed