GitLab

This relands r561423.
Original code review: https://chromium-review.googlesource.com/1069956

The original patch had a problem because the shadow page was created
before UnguessableToken. The fix is to not create the shadow page early,
which was only needed in an earlier patchset to get the task runner
from it.

Bug: 839982, 846545
Change-Id: Id6545d9d86a3a56a04e7f83a840d9acddd55f51b
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
TBR: kinuko, dcheng
Reviewed-on: https://chromium-review.googlesource.com/1073037
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561793}