Aligns console.count() behavior with spec, which says the default label
should be "default" when the label provided is not defined.

Bug: chromium:700624
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie58af210d300ef3151082b23187dd18e356f5de8
Reviewed-on: https://chromium-review.googlesource.com/780620
Commit-Queue: Erik Luo <luoe@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49804}