infra: Switch android-marshmallow-x86-rel to builderfull.

To hopefully help avoid high pending times -> low cache hit -> increased
build durations.

crrev.com/i/3402719 allocates the machines for this builder.

Bug: 1140727
Change-Id: Ieb64b7e2a9ea829540d42878d0adb3f8b6c98eb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540922Reviewed-by: Haiyang Pan <hypan@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827856}

infra/config/generated/cr-buildbucket.cfg

@@ -24432,7 +24432,7 @@ buckets {
       name: "android-marshmallow-x86-rel"
       swarming_host: "chromium-swarm.appspot.com"
       swarming_tags: "vpython:native-python-wrapper"
-      dimensions: "builderless:1"
+      dimensions: "builder:android-marshmallow-x86-rel"
       dimensions: "cores:16"
       dimensions: "cpu:x86-64"
       dimensions: "os:Ubuntu-16.04"

infra/config/subprojects/chromium/try.star

@@ -346,6 +346,7 @@ try_.chromium_android_builder(
 try_.chromium_android_builder(
     name = "android-marshmallow-x86-rel",
     branch_selector = branches.STANDARD_MILESTONE,
+    builderless = not settings.is_master,
     cores = 16,
     goma_jobs = goma.jobs.J300,
     ssd = True,