v8binding: Create callback from v8::Object when [TreatNonObjectAsNull]

Allows to create an IDL callback function from v8::Object when
[TreatNonObjectAsNull] is specified.  This is a follow-up of
https://crrev.com/c/1206790

Bug: 872138
Change-Id: I8913c97983acfc504bb6faa48e367ceec0b25be3
Reviewed-on: https://chromium-review.googlesource.com/1223952
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590960}
3 files changed