Make update-flaky-tests work for debug builders as well.

We were just looping over port names before, which will default
us to only use release builds. Also, that's wrong anyways because
there's no guarantee that a port actually runs on a builder yet.
Instead, loop over the builders. We assume throughout the codebase
that these builders upload results to the flakiness dashboard,
so it's correct to use the builders list here.

Review URL: https://codereview.chromium.org/326483002

git-svn-id: svn://svn.chromium.org/blink/trunk@175811 bbb929c8-8fbe-4397-9dbb-9b2b20218538