First cut at run_mksnapshot action for gn

BUG=none
R=brettw@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/293363009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21618 ce2b1a6d-e550-0410-aec6-3dcde31c8c00