[infra/config] switch to use xenial for some builders

This CL switches following builders.
- chromeos-amd64-generic-asan-rel (CI)
- chromeos-arm-generic-rel (CI)
- linux-blink-heap-concurrent-marking-tsan-rel (CI/CQ)

Bug: 954450
Change-Id: I993d0d8b14d5dacc4e96906b55b1ead698cf34ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710895Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679888}

infra/config/cr-buildbucket.cfg

@@ -1351,6 +1351,7 @@ buckets {
     builders {
       name: "chromeos-amd64-generic-asan-rel"
       mixins: "chromeos-ci"
+      mixins: "linux-xenial"
       mixins: "builderless"
     }
 
@@ -1379,6 +1380,7 @@ buckets {
     builders {
       name: "chromeos-arm-generic-rel"
       mixins: "chromeos-ci"
+      mixins: "linux-xenial"
       mixins: "builderless"
     }
 
@@ -1606,6 +1608,7 @@ buckets {
       mixins: "fyi-ci"
       mixins: "linux"
       mixins: "builderless"
+      mixins: "linux-xenial"
     }
 
     builders {
@@ -4073,6 +4076,7 @@ buckets {
     builders {
       mixins: "builderless"
       mixins: "linux-try"
+      mixins: "linux-xenial"
       name: "linux-blink-heap-concurrent-marking-tsan-rel"
     }
     builders {