luci: switch 6 builders to builderless machine allocation.

Follow-up to comments on crrev.com/i/1164251

Bug: 947555
Change-Id: I83417737552c38d47e5ec6a3268179fb1ac07973
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562766
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649759}

infra/config/cr-buildbucket.cfg

@@ -679,6 +679,12 @@ builder_mixins {
   }
 }
 
+builder_mixins {
+  name: "builderless"
+  auto_builder_dimension: NO
+  dimensions: "builderless:1"
+}
+
 buckets {
   name: "ci"
   acl_sets: "ci"
@@ -711,6 +717,7 @@ buckets {
     builders {
       name: "Android arm Builder (dbg)"
       mixins: "android-ci"
+      mixins: "builderless"
       dimensions: "os:Ubuntu-14.04"
       execution_timeout_secs: 14400  # 4h
     }
@@ -718,6 +725,7 @@ buckets {
     builders {
       name: "Android arm64 Builder (dbg)"
       mixins: "android-ci"
+      mixins: "builderless"
       mixins: "goma-many-jobs-for-ci"
       dimensions: "os:Ubuntu-14.04"
       execution_timeout_secs: 14400  # 4h
@@ -865,6 +873,7 @@ buckets {
     builders {
       name: "Android x64 Builder (dbg)"
       mixins: "android-ci"
+      mixins: "builderless"
       dimensions: "os:Ubuntu-14.04"
       execution_timeout_secs: 14400  # 4h
     }
@@ -872,6 +881,7 @@ buckets {
     builders {
       name: "Android x86 Builder (dbg)"
       mixins: "android-ci"
+      mixins: "builderless"
       dimensions: "os:Ubuntu-14.04"
     }
 
@@ -1044,6 +1054,7 @@ buckets {
     builders {
       name: "chromeos-amd64-generic-asan-rel"
       mixins: "chromeos-ci"
+      mixins: "builderless"
     }
 
     builders {
@@ -1223,6 +1234,7 @@ buckets {
       name: "linux-blink-heap-concurrent-marking-tsan-rel"
       mixins: "fyi-ci"
       mixins: "linux"
+      mixins: "builderless"
     }
 
     builders {