Also cast recursedeps to a set.  We'll continue
to support using a list literal form in DEPS for
now.

R=iannucci@chromium.org,agable@chromium.org
BUG=393000

Review URL: https://codereview.chromium.org/385123008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@283474 0039d316-1c4b-4281-b951-d872f2087c98