Add --add-new-files option to download-baselines.py script

This will allow the user to determine whether the script downloads just
updates for baselines that are already checked in, or all available baselines.

BUG=436
addresses http://code.google.com/p/skia/issues/detail?id=436 ('add "update" flag to download-baselines script')
Review URL: http://codereview.appspot.com/5523050

git-svn-id: http://skia.googlecode.com/svn/trunk@2993 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed