desktop-pwas: Move placeholder app creation logic into BookmarkAppInstallationTask

This is part of simplifying the logic in PendingAppManager.

Temporarily pass the result of loading the URL to the installation task.
A follow up CL will move the loading logic into the task so the argument
will be removed.

Bug: 916381
Change-Id: I45367c08a3195e2bfb4d0e83e36350b67e61c84b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616709
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661261}
5 files changed