[w3c/manifest] Breaking: Ignore any icon with no recognised purpose. (#741)

Closes #728.

This change (choose one):

* [X] Breaks existing normative behavior (please add label "breaking")
* [ ] Adds new normative behavior
* [ ] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [ ] Is a "chore" (metadata, formatting, fixing warnings, etc).

Implementation commitment (delete if not making normative changes):

* [ ] Safari (link to issue)
* [X] Chrome (https://crbug.com/905561)
* [ ] Firefox (link to issue)
* [ ] Edge (public signal)

Commit message:

Breaking: Ignore any icon with no recognised purpose.

Previously, if all of an icon's purposes were unrecognised, it could be
used for any purpose. This causes forwards-compatibility issues, since
if we add a new purpose (such as the recently added 'maskable'), old
user agents will interpret icons with that new purpose as 'any', which
is not intended.

Icons with no stated purpose, or whose purpose is the empty string, are
still treated as 'any'.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/741

-- Commit Summary --

  * Breaking: Ignore any icon with no recognised purpose.

-- File Changes --

    M index.html (69)

-- Patch Links --

https://github.com/w3c/manifest/pull/741.patch
https://github.com/w3c/manifest/pull/741.diff

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