v8binding: Support [TreatNonObjectAsNull] extended attribute.

When I first implemented IDL callback function, there was no use
case of [TreatNonObjectAsNull], and I didn't support it.  However,
now we're going to implement EventHandler of HTML, which uses
[TreatNonObjectAsNull], based on IDL callback function.  So, this
patch supports [TreatNonObjectAsNull].

Bug: 872138
Change-Id: I2c4b7bd3b321e23cf27f4442ffa72ff76dc15e85
Reviewed-on: https://chromium-review.googlesource.com/1206790
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589124}
23 files changed