Add mb configuration for google-chrome-chromeos.

Bug: 928475
Change-Id: I59d7ad45877439d05ce0ea509fa78a195b24284a
Reviewed-on: https://chromium-review.googlesource.com/c/1468586Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632055}

tools/mb/mb_config.pyl

@@ -647,6 +647,10 @@
       'win_angle_x64_rel_ng': 'gpu_fyi_tests_release_trybot',
     },
 
+    'tryserver.chromium.chrome': {
+      'google-chrome-chromeos': 'official_goma_chromeos',
+    },
+
     'tryserver.chromium.chromiumos': {
       # TODO(crbug.com/913750): Enable DCHECKS on the two amd64-generic bots
       # when the PFQ has it enabled.