Add a new Chrome OS factory class, ChromitePlusFactory.
Add a new Chrome OS factory class, ChromitePlusFactory.

This CL introduces a new ChromiteFactory that checks out an additional
repo and runs a script from said repo. This is useful/necessary when
you have an internal/separate repo that uses chromite.

BUG=None
TEST=Set up a local slaves with new factory and ran it with chromeos
master.

Review URL: https://codereview.chromium.org/84253002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@238208 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build