Implement manifest icon downloader
This CL unifies code used by app banners and shortcut
helper into one class. It further implements icon scaling
which fixes the issue of Intent overflow on Android when
there is a bad manifest/only a big icon is provided.
It also refactors code for a future CL which will implement
downloading of another icon for installed webapps
splashscreen on Android.
BUG=513666
Review URL: https://codereview.chromium.org/
1261143004
Cr-Commit-Position: refs/heads/master@{#344787}