Switch downstream linux-chromeos tests back to trusty.

Partial revert of crrev.com/c/1689513.

chrome-swarming doesn't have xenial bots in the main chrome test pool:
https://ci.chromium.org/p/chrome/builders/ci/linux-chromeos-google-rel/4291

# Notry'ing to fix bots.
NOTRY=true

Bug: 954450
Change-Id: Ifbfb4c4dc099184f24714ac07a4c501830b75e5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692823
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675701}

testing/buildbot/waterfalls.pyl

@@ -44,7 +44,9 @@
         ],
         'mixins': [
           'chrome-swarming-pool',
-          'linux-xenial',
+          # TODO(crbug.com/954450): Switch to xenial when chrome-swarming has
+          # xenial bots.
+          'linux-trusty',
         ],
         'test_suites': {
           'gtest_tests': 'linux_chromeos_gtests',