Eliminated test flakiness for script-priorities

The async scripts execute in non-deterministic order so I removed their
debug output.  The only thing we really care about testing in the
script-priorities layout test is the request priority which is
determined and logged deterministically at parse time (and still
captured).

BUG=413604

Review URL: https://codereview.chromium.org/555463006

git-svn-id: svn://svn.chromium.org/blink/trunk@181919 bbb929c8-8fbe-4397-9dbb-9b2b20218538