Include USB printers in printer list as "provisional" devices.

This patch starts including a list of connected printers supported by
printerProvider apps in the normal list of extension printers returned
to the print preview WebUI. These printers are given "provisional" IDs
because the devices are not yet known to the apps themselves.

BUG=468955

Review URL: https://codereview.chromium.org/1153173002

Cr-Commit-Position: refs/heads/master@{#331886}
6 files changed