This allows specificying that a target OS list should override the base
list instead of adding to it. This is useful for iOS builds, where the
desired behavior is to pull only the iOS deps, not the union of the Mac
and iOS deps.


Review URL: https://chromiumcodereview.appspot.com/11363036

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@165671 0039d316-1c4b-4281-b951-d872f2087c98