roll_deps.py: Make default_bots_list a module-level variable

This allows us to import the script rather than run it in a sub-shell.

BUG=skia:
R=halcanary@google.com

Review URL: https://codereview.chromium.org/140973003

git-svn-id: http://skia.googlecode.com/svn/trunk@13200 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed