Reland of https://codereview.chromium.org/268373003/ with a fix (wraps the
url with str())
https://codereview.chromium.org/268393002/ (bot_update root@revision support) is dependent on this

BUG=370503
TEST= gclient sync --output-json ~/tmp/asdfasdf.json on both an SVN and Git checkout

Review URL: https://codereview.chromium.org/277553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@269469 0039d316-1c4b-4281-b951-d872f2087c98