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Web App Manifest icons can have the MASKABLE purpose specifying they
were designed with all of their important content sufficiently far from
the edges to allow the User Agent to apply a mask [1].

This CL just adds code to read in that purpose, which will be used in
upcoming CLs.

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

Bug: 904354
Change-Id: Ib62eb7af9226d14199d0d74615cb5bbf2666abda
Reviewed-on: https://chromium-review.googlesource.com/c/1329792
Commit-Queue: Peter Conn <peconn@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608448}