[test] switch to use xenial for UBSanVptr Linux builder

The builder has 10 hour hourly resource usage in p99.
http://shortn/_1XQzVGUb76

Failure of linux_chromium_ubsan_rel_ng is not xenial specific.
(crbug.com/989936, crbug.com/989935)

Bug: 954450
Change-Id: If101be573202e0dffd6d2e5f902ed420f1c2c1a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730051
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683197}

testing/buildbot/waterfalls.pyl

@@ -974,7 +974,7 @@
       },
       'UBSanVptr Linux': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           # no "_and_gl":gl_unittests doesn't pass yet,