Add linux-chromeos-compile-dbg to the CQ at 50% experiment.

Hopefully should have enough bots:
https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/linux-chromeos-compile-dbg

R=jbudorick, dpranke

Bug: 902468
Change-Id: I9273d96ede4c056d8bd4b936f5f843176bac087a
Reviewed-on: https://chromium-review.googlesource.com/c/1340536Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609025}

infra/config/branch/cq.cfg

@@ -101,6 +101,10 @@ verifiers {
         path_regexp: "third_party/WebKit/LayoutTests/flag-specific/enable-blink-gen-property-trees/.+"
       }
       builders { name: "linux-chromeos-rel" }
+      builders {
+        name: "linux-chromeos-compile-dbg"
+        experiment_percentage: 50
+      }
       builders { name: "linux_chromium_asan_rel_ng" }
       builders { name: "linux_chromium_compile_dbg_ng" }
       builders {