GM: make behavior of make_filename() more late-binding

This will be needed to add the option of writing out files with
checksum-based filenames.

R=scroggo@google.com

Review URL: https://codereview.chromium.org/15415003

git-svn-id: http://skia.googlecode.com/svn/trunk@9215 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed