Fix line numbers not matching up with code in Firefox.

Firefox has had broken line number alignment for a while now, if this is a bug with Firefox they don't seem to want to fix it since its been there from 46 through 49. This fixes it.

Before:

After:

Fixes #20202 (closed).

Safari/Chrome work the same as before, Snippets seems to work fine still as well.

Page tested: http://localhost:3000/gitlab-org/gitlab-ce/blob/master/app/models/project.rb

cc: @annabeldunstone