Use nullable interface for RequestInit AbortSignal

R=kinuko@chromium.org, peria@chromium.org, ricea@chromium.org, yhirano@chromium.org, yoav@yoav.ws

Bug: 855968
Change-Id: I96be3b769dd75300553f639ca5fa8158771bde2c
Reviewed-on: https://chromium-review.googlesource.com/1137930
Commit-Queue: Dominic Farolino <domfarolino@gmail.com>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576204}
3 files changed