If object contains undefined property then JSON.stringify will skip it,
if array contains undefined as property then JSON.stringify will censor
it to null. [1]

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify

R=alph@chromium.org

Bug: none
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iafa9d2828d264d89b26675b0e194ad0bfc4621fc
Reviewed-on: https://chromium-review.googlesource.com/834669Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50220}