[infra/pgo] mb configs for the pgo beta/stable trybots tryserver

MB configs for the beta/stable trybots for PGO. Missed tryserver
definition.

Bug: 1081356,1085606
Change-Id: Ib17b5cab291ac9cbf8769c3622cb2793a60e584e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214002
Auto-Submit: Jeff Yoon <jeffyoon@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771860}

tools/mb/mb_config.pyl

@@ -778,9 +778,15 @@
       'linux-chrome': 'official_goma',
       'linux-chromeos-chrome': 'official_goma_chromeos_include_unwind_tables',
       'mac-chrome': 'official_goma',
+      'mac-chrome-beta': 'official_goma',
+      'mac-chrome-stable': 'official_goma',
       'win-celab-try-rel': 'official_celab_release_bot',
       'win-chrome': 'official_goma_x86',
+      'win-chrome-beta': 'official_goma_x86',
+      'win-chrome-stable': 'official_goma_x86',
       'win64-chrome': 'official_goma_x64',
+      'win64-chrome-beta': 'official_goma_x64',
+      'win64-chrome-stable': 'official_goma_x64',
     },
 
     'tryserver.chromium.chromiumos': {