[testing] use xenial for test from chromeos asan builder

Test from this builder consumes around the following machine hour
* p99 CI: 38.27611111111111
* p90 CI: 34.07888888888889
* p50 CI: 32.31194444444444
* p99 CQ: 36.07277777777778
* p90 CQ: 35.85166666666667
* p50 CQ: 28.64027777777778

in chrome pool in recent 30 days.

I took the stats from
http://shortn/_vS3Df0uFD0

Bug: 954450
Change-Id: I050e133192423c1bb734e612c9a8be2417259a22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712704Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680314}

testing/buildbot/waterfalls.pyl

@@ -3672,7 +3672,7 @@
       },
       'Linux Chromium OS ASan LSan Tests (1)': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'linux_chromeos_gtests',