[infra/config] switch msan builders to xenial builderless

Bots are added by
https://chrome-internal-review.googlesource.com/c/infradata/config/+/1506829

Bug: 954450, 977061, 956776
Change-Id: Ibffe21b019a0bb90f3f28869a7592f630792e3d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714305Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680280}

infra/config/cr-buildbucket.cfg

@@ -2525,9 +2525,10 @@ buckets {
     }
     builders {
       name: "MSAN Release (chained origins)"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
       mixins: "goma-rbe-prod"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "TSAN Release"
@@ -2565,10 +2566,11 @@ buckets {
     }
     builders {
       name: "Libfuzzer Upload Linux MSan"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
       mixins: "libfuzzer"
       mixins: "goma-rbe-prod"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "TSAN Debug"
@@ -2734,8 +2736,9 @@ buckets {
     }
     builders {
       name: "Linux MSan Tests"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "memory-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "Mac ASan 64 Tests (1)"
@@ -2771,9 +2774,10 @@ buckets {
     }
     builders {
       name: "MSAN Release (no origins)"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
       mixins: "goma-rbe-prod"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "Linux Chromium OS ASan LSan Builder"
@@ -2948,8 +2952,9 @@ buckets {
     }
     builders {
       name: "Linux MSan Builder"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "memory-ci-goma-rbe-prod"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "ASan Release Media (32-bit x86 with V8-ARM)"