Change download-baselines script to download images from skia-autogen SVN repo

Change download-baselines script to download images from skia-autogen SVN repo

I have copied the old version (which downloads the images from the buildbots
directly, but only works with our Mac buildbots) to download-baselines-old,
so we can use either version during a transition period.

Another difference: the new version sets the mimetype property of all image
files in the baseline_subdir, even those that have not changed.

BUG=386
http://code.google.com/p/skia/issues/detail?id=386 ('make buildbots write out RunGM image results to a browsable directory')
Review URL: https://codereview.appspot.com/5544056

git-svn-id: http://skia.googlecode.com/svn/trunk@3058 2bbb7eff-a529-9590-31e7-b0007b416f81