Improve setup_dev_packages so that it can run incrementally.
Improve setup_dev_packages so that it can run incrementally.

BUG=none
TEST=ran to install latest libchrome, etc.

Change-Id: I7d48a77b1528f5fb4ea1ba5b7f52a9a248f71c84

Review URL: http://codereview.chromium.org/3169041

marineam/update_engine,marineam/update_engine,mjg59/update_engine,endocode/update_engine,endocode/update_engine,mjg59/update_engine,marineam/update_engine,mjg59/update_engine,marineam/update_engine,endocode/update_engine