[infra/config] switch to use xenial builderless for lkgr builders

bots are added in
https://chrome-internal-review.googlesource.com/c/infradata/config/+/1397329

Bug: 954450, 956776
Change-Id: I1e7d92c4ee23e6d96cc327410b1cb8083ceea101
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662248
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669995}

infra/config/cr-buildbucket.cfg

@@ -2508,8 +2508,9 @@ buckets {
     }
     builders {
       name: "TSAN Release"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "Linux CFI"
@@ -2546,13 +2547,15 @@ buckets {
     }
     builders {
       name: "TSAN Debug"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "UBSan vptr Release"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "Mac ASAN Release"
@@ -2601,18 +2604,21 @@ buckets {
     }
     builders {
       name: "UBSan Release"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "ChromiumOS ASAN Release"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "ASAN Release Media"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "win-archive-dbg"
@@ -2666,8 +2672,9 @@ buckets {
 
     builders {
       name: "ASan Debug (32-bit x86 with V8-ARM)"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "Linux remote_run Tester"
@@ -2711,8 +2718,9 @@ buckets {
     }
     builders {
       name: "ASAN Debug"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "Mac deterministic"
@@ -2785,8 +2793,9 @@ buckets {
     }
     builders {
       name: "ASAN Release"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "Libfuzzer Upload Linux ASan Debug"
@@ -2893,8 +2902,9 @@ buckets {
     }
     builders {
       name: "ASan Release Media (32-bit x86 with V8-ARM)"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "mac-hermetic-upgrade-rel"
@@ -2970,8 +2980,9 @@ buckets {
     }
     builders {
       name: "ASan Release (32-bit x86 with V8-ARM)"
-      dimensions: "os:Ubuntu-14.04"
       mixins: "fuzz-ci"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "WebKit Win10"