switch trusty -> xenial for linux-chromeos-rel

This is step 2 written in
https://chrome-internal-review.googlesource.com/c/infradata/config/+/1311342

Bug: 954450
Change-Id: I5eff9cf0073fc5ef44d4225f5e2286d490983872
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621409
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663480}
3 files changed