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This is a follow-up CL for https://crrev.com/c/1597496 that updates
manifest.mojom to make the start_url, icons, related_applications,
splash_screen_url, and scope members non-optional in order to
follow the web app manifest spec
(https://www.w3.org/TR/appmanifest/#processing).

Bug: 704441
Change-Id: I92c6d8c220e05c9394d62a06ca4450191f1c552d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1633695
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664499}