Adds url to gclient json (try 2)

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

gclient.py

@@ -1746,6 +1746,7 @@ def CMDsync(parser, args):
       slns[normed] = {
           'revision': d.got_revision,
           'scm': d.used_scm.name if d.used_scm else None,
+          'url': str(d.url) if d.url else None,
       }
     with open(options.output_json, 'wb') as f:
       json.dump({'solutions': slns}, f)