Make hamburger key a "system key" so can be used on Crostini windows

The hamburger key (opens the system tray) currently is sent to Crostini
windows, but it should be handled by Chrome OS in all cases. The key
works fine with Arc++ windows as they don't ack the key, but it seems
better to not pass it to the window at all, so this CL makes the key a
system key.

Bug: 1022669
Change-Id: Ib70d8d1181ac02aa537722e3a483d2cbdb581f43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3255122
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#942519}