Move commit list out its own model.

This CL creates a model to hold information on the commit lists. This is
in prepartion for adding the ability to expand the commit list.

BUG=399715
NOTRY=true

Review URL: https://codereview.chromium.org/464963003

git-svn-id: svn://svn.chromium.org/blink/trunk@180385 bbb929c8-8fbe-4397-9dbb-9b2b20218538