Re-enable networkState_during_progress.html but mark as flaky.

This CL also fixes the -expected file with the correct expectations (PASS),
though this test rarely passes.

As noted in the bug, the test has been confirmed correct, and we need to fix
our behavior. At that time, we can remove the TestExpectations entry.

BUG=306249

Review URL: https://codereview.chromium.org/204003004

git-svn-id: svn://svn.chromium.org/blink/trunk@169521 bbb929c8-8fbe-4397-9dbb-9b2b20218538