Mark two frameNavigation tests which recently started flaking

These were reported for spurious try-job rejections.

TBR=dcheng
BUG=374270

Review URL: https://codereview.chromium.org/290143003

git-svn-id: svn://svn.chromium.org/blink/trunk@174190 bbb929c8-8fbe-4397-9dbb-9b2b20218538