Replace undefined assert_undefined function.

assert_undefined(v) was a convenience function for assert_equals(v, null) that
was lost when merging player-unresolved-times.html with the W3C startTime.html
test file in http://crrev.com/2141853002

BUG=623437

Review-Url: https://codereview.chromium.org/2147643003
Cr-Commit-Position: refs/heads/master@{#405055}
2 files changed