Add support for Manifest.icons.sizes

This is re-implementing the HTML icon sizes parsing algorithm
given that it is missing a re-usable place for it. There is
a follow-up bug opened at http://crbug.com/416477

BUG=366145

Review URL: https://codereview.chromium.org/591073002

Cr-Commit-Position: refs/heads/master@{#296386}
4 files changed