Make download_baselines.py download *all* platforms, if none are specified.

Simplifies the Skia rebaselining process... updated the --help pages in both scripts, too.

BUG=http://code.google.com/p/skia/issues/detail?id=486
Review URL: https://codereview.appspot.com/6016045

git-svn-id: http://skia.googlecode.com/svn/trunk@3664 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed