Without this change, if an entry is removed from a DEPS file, it won't be
effectively deleted. Specify --delete_unversioned_trees so the dependency is
properly deleted.

TBR=szager@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/14531006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@197081 0039d316-1c4b-4281-b951-d872f2087c98