Also, add support for directly setting the public read flag on uploaded files.

The support for tar.gz allows the uploaded bundle to be a single tar.gz file instead of a lot of individual files using the directory support already in. The benefit here is that it is much easier to update the dependency. Simply clean out the existing files, copy in the new ones, create a tar.gz file, with the same name as the directory + 'tar.gz'. If the directory name and file name does not match up we will not clean up existing artifacts on download (i.e., there can be left over files after extracting).

I am doing this because I am moving a bunch of the dart dependencies to gcs, and a lot of our dependencies is much easier to manage with this in. If you don't like this, I can simply wrap the download script in another python script and do the logic there, but this may be handy for other people as well.

Review URL: https://codereview.chromium.org/807463005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295872 0039d316-1c4b-4281-b951-d872f2087c98