[infra/config] use xenial for webrtc tester

This will be submit after crbug.com/985705 for
https://ci.chromium.org/p/chromium/builders/webrtc/WebRTC%20Chromium%20Linux%20Tester

Bug: 954875
Change-Id: I486cb6de5604f00cdfb4a3b9079647e8c21be3d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1757417
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689176}

infra/config/cr-buildbucket.cfg

@@ -4934,7 +4934,7 @@ buckets {
     }
     builders {
       name: "WebRTC Chromium Linux Tester"
-      mixins: "linux-trusty"
+      mixins: "linux-xenial"
     }
 
     # Mac