Implement app banner info bars on desktop.
This CL extends the app banner implementation to work on desktop,
enabled with the flag --enable-add-to-shelf. With this flag active,
users will be prompted with an infobar to add a site to their shelf
once they have engaged with it to some substantive amount (subject to
heuristics currently implemented for the mobile add to homescreen
banner).
Note: the icon used is a placeholder to be updated later.
BUG=491001
Review URL: https://codereview.chromium.org/
1161233005
Cr-Commit-Position: refs/heads/master@{#332988}