crostini: Show a toast when a user tries to combine VK + Crostini

Instead of logging a console message, we now show a toast to the user
for 5 seconds the first time (per session) they focus a Crostini app
while in tablet mode.

./out/Default/unit_tests
for one and only one toast

Bug: chromium:962848
Test: autoninja -C out/Default chrome unit_tests &&
Test: Play around with tablet mode and Crostini on an Eve device, check
Change-Id: Ie70a847372f2a65d3f71fc39e29cb1165992ab3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743232
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#685081}
3 files changed