[UserTiming] Fix mark-measure-return-null test

The test was incorrect per L2 spec: a void method 'returns' an undefined
value, not a null object. This CL changes the test to show how to
perform feature detection and compares L2 and L3 behavior.

Bug: 914441
Change-Id: Id7173b2693d5bd42013dcdd25266d64fa8956e6a
Reviewed-on: https://chromium-review.googlesource.com/c/1377129
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616712}
4 files changed