Mark http/tests/w3c/webperf/submission/Intel/user-timing/test_user_timing_mark.html as flaky in debug

TBR=simonjam@chromium.org
NOTRY=TRUE
BUG=398295

Review URL: https://codereview.chromium.org/426953003

git-svn-id: svn://svn.chromium.org/blink/trunk@179136 bbb929c8-8fbe-4397-9dbb-9b2b20218538