Remove layout tests from 'mac-rel'

This CL removes webkit_layout_tests from 'Mac10.13 Tests', which will
affect mac-rel. The bot is running out of capacity, and there's no clear
solution. This should hopefully help the bot be much faster.

https://ci.chromium.org/p/chromium/builders/luci.chromium.try/mac10.13-blink-rel
still runs webkit_layout_tests, and they're still run on the CI bots.

R=dpranke

No-Try: True
Bug: 943804
Change-Id: Ifb15899fc25fee3de0107d9a99f10671d71e8942
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531705
Reviewed-by: Nico Weber <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Stephen Martinis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#642599}
diff --git a/testing/buildbot/generate_buildbot_json.py b/testing/buildbot/generate_buildbot_json.py
index 567882e..f48eca5 100755
--- a/testing/buildbot/generate_buildbot_json.py
+++ b/testing/buildbot/generate_buildbot_json.py
@@ -939,6 +939,8 @@
       # chromium, due to https://crbug.com/878915
       'win-dbg',
       'win32-dbg',
+      # chromium.mac, see https://crbug.com/943804
+      'mac-dummy-rel',
       # Defined in internal configs.
       'chromeos-amd64-generic-google-rel',
     ]