Mark test_user_timing_measure_navigation_timing.htm as flaky

BUG=395127
TBR=tonyg@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/398423005

git-svn-id: svn://svn.chromium.org/blink/trunk@178445 bbb929c8-8fbe-4397-9dbb-9b2b20218538