Some User Timing tests are flaky

[foolip]

https://github.com/web-platform-tests/wpt/pull/14409#issuecomment-445753408 lists a number of tests in user-timing/ that were flaky in Firefox:

Test	Subtest	Results	Messages
/user-timing/measure_associated_with_navigation_timing.html	Second measure of current mark to navigationStart should be negative value.	FAIL: 1/10, PASS: 9/10	assert_true: Second measure of current mark to navigationStart should be negative value. expected true got false
/user-timing/measure_associated_with_navigation_timing.html	Measure from domComplete event to most recent mark "a" should have longer duration.	FAIL: 1/10, PASS: 9/10	assert_true: Measure from domComplete event to most recent mark "a" should have longer duration. expected true got false
/user-timing/measure_associated_with_navigation_timing.html	Measure from most recent mark to navigationStart should have longer duration.	FAIL: 1/10, PASS: 9/10	assert_true: Measure from most recent mark to navigationStart should have longer duration. expected true got false

Suggested method to investigate:

1. Add a trailing whitespace or other no-op to all tests in user-timing/ in a test PR
2. Check which were flaky on Taskcluster
3. Fix them :)

[foolip]

Notifying suggested reviewers for user-timing: @plehegar @igrigorik @toddreifsteck