Make linux_chromium_compile_dbg_ng builderfull on master branch only.

This is the first in a series of changes to make all ToT linux CQ
builders builderfull.

crrev.com/i/3351753 allocates the machines for this builder, so this
shouldn't be submitted before that.

Bug: 1140727
Change-Id: I976f419b4be7249534b210e05151694734acbe96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503391Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821817}

infra/config/generated/cr-buildbucket.cfg

@@ -30111,12 +30111,11 @@ buckets {
       name: "linux_chromium_compile_dbg_ng"
       swarming_host: "chromium-swarm.appspot.com"
       swarming_tags: "vpython:native-python-wrapper"
-      dimensions: "builderless:1"
+      dimensions: "builder:linux_chromium_compile_dbg_ng"
       dimensions: "cores:8"
       dimensions: "cpu:x86-64"
       dimensions: "os:Ubuntu-16.04"
       dimensions: "pool:luci.chromium.try"
-      dimensions: "ssd:0"
       exe {
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"
         cipd_version: "refs/heads/master"

infra/config/subprojects/chromium/try.star

@@ -1083,6 +1083,7 @@ try_.chromium_linux_builder(
 try_.chromium_linux_builder(
     name = "linux_chromium_compile_dbg_ng",
     branch_selector = branches.STANDARD_MILESTONE,
+    builderless = not settings.is_master,
     caches = [
         swarming.cache(
             name = "builder",