The original change didn't handle the case where the injected code to
click "Cancel" button was run after the dialog page has fully loaded. In
this case, the DOMContentLoaded event has already fired.

BUG=808300

No-try: true
Change-Id: Ibf8408a7a44ef8bc8029764d31bc9a5171119cc8
Reviewed-on: https://chromium-review.googlesource.com/c/1350444
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610812}