MB: Add Chromium Linux Goma GCE Staging

BUG=728482

Review-Url: https://codereview.chromium.org/2913353003
Cr-Commit-Position: refs/heads/master@{#476226}

tools/mb/mb_config.pyl

@@ -221,6 +221,7 @@
       'Chromium Linux Goma Staging': 'release_bot',
       'Chromium Mac Goma Staging': 'release_bot',
       'CrWinGomaStaging': 'release_bot_x86_minimal_symbols',
+      'Chromium Linux Goma GCE Staging': 'release_bot',
     },
 
     'chromium.gpu': {