Mark http/tests/navigation/back-to-redirect-with-frame.php as flaky.

BUG=339597
TBR=caseq

Review URL: https://codereview.chromium.org/136753011

git-svn-id: svn://svn.chromium.org/blink/trunk@166173 bbb929c8-8fbe-4397-9dbb-9b2b20218538