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Originally, the installability check verified whether or not the
manifest scope is within a service worker's scope. Today, we want to
also check if the page specified by `start_url` in a manifest has a
service worker and works well offline.

Bug: 1157242, 965802
Change-Id: Ibe0a8a76a34bccecd461d9b106590e00807b531f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2583674
Commit-Queue: Asami Doi <asamidoi@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836936}