Only show Usb sharing notifications when Crostini is enabled.

If other guest OSes are supported in future, we will refine this rule.

Bug: 948913
Change-Id: I2fb3db1a6cb690237a2c6896826311e895862531
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559412
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Auto-Submit: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649359}