This CL changes the check for base URL to a query to the json endpoint.
This makes the test pass on the trybot.

BUG=353718

Review URL: https://codereview.chromium.org/1160273005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295538 0039d316-1c4b-4281-b951-d872f2087c98