[testing] use xenial for test from chromeos msan builder

Test from this builder consumes around the following machine hour
* p99 CI: 67.32944444444445
* p90 CI: 38.772222222222226
* p50 CI: 36.01555555555556
* p99 CQ: 61.65472222222222
* p90 CQ: 51.79527777777778
* p50 CQ: 33.22638888888889

in chrome pool in recent 30 days.

I took the stats from
http://shortn/_MnHq3A4dxc

some test failure is not xenial specific (crbug.com/988997).

I will submit this with
https://chrome-internal-review.googlesource.com/c/infradata/config/+/1543036

Bug: 954450
Change-Id: I4c69fe4c9451958a519685d0f5209c75699e3b8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710897Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682246}

testing/buildbot/waterfalls.pyl

@@ -3699,7 +3699,7 @@
       },
       'Linux ChromiumOS MSan Tests': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
           'x86-64',
         ],
         'test_suites': {