Change depot_tools presubmit to use infra rietveld from head. Also disable newly added backends by specifying app.yaml explicitly.

BUG=481568

Review URL: https://codereview.chromium.org/1110403003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295099 0039d316-1c4b-4281-b951-d872f2087c98