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Web App Manifests allow icons to be specified with the "maskable"
purpose [1]. These icons are designed with enough space on the sides
to allow the user agent to mask them (eg, cut the icon into a circle).

On devices that support this, attempt to load a maskable icon.

[1] https://w3c.github.io/manifest/#purpose-member

Bug: 904354
Change-Id: I08c404d7b956e678b37f995e9ae0d3f13a5a49fe
Reviewed-on: https://chromium-review.googlesource.com/c/1333778
Commit-Queue: Peter Conn <peconn@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608758}