[mac] Add build configuration for mac_chromium_10.10.

Bug: 764076
Change-Id: I759bc1950ff1022535364d605dbe69300d653365
No-Equivalent-Builders: true
Reviewed-on: https://chromium-review.googlesource.com/661478
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501116}

tools/mb/mb_config.pyl

@@ -616,6 +616,7 @@
       'ios-simulator-eg': 'ios',
       'ios-simulator-cronet': 'ios',
       'ios-simulator-xcode-clang': 'ios',
+      'mac_chromium_10.10': 'gpu_tests_release_trybot',
       'mac_chromium_10.10_macviews': 'mac_views_browser_release_trybot',
       'mac_chromium_10.12_rel_ng': 'gpu_tests_release_trybot',
       'mac_chromium_archive_rel_ng': 'release_bot_mac_strip',