Mark test_user_timing_measure_navigation_timing.htm as flaky

Fixes incorrect marking as flaky in
https://codereview.chromium.org/398423005, which forgot that the
FlakyTests file is auto-generated.

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

Review URL: https://codereview.chromium.org/400223003

git-svn-id: svn://svn.chromium.org/blink/trunk@178452 bbb929c8-8fbe-4397-9dbb-9b2b20218538