Add mb_config entry for android_optional_gpu_tests_rel.

tools/mb/mb_config.pyl

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   # This is a map of buildbot master names -> buildbot builder names ->
   # config names (where each config name is a key in the 'configs' dict,
   # below). MB uses this dict to look up which config to use for a given bot.
   'masters': {
     # Take care when changing any of these builders to ensure that you do not
@@ skipping 667 matching lines @@
       'android_compile_dbg': 'swarming_android_gn_debug_trybot',
       'android_compile_mips_dbg': 'android_gn_debug_trybot_mipsel',
       'android_compile_rel': 'swarming_android_gn_release_trybot',
       'android_compile_x64_dbg': 'android_gn_debug_trybot_x64',
       'android_compile_x86_dbg': 'android_gn_debug_trybot_x86',
       'android_coverage': 'android_gn_debug_trybot_java_coverage',
       # TODO(crbug/597596): Switch this back to debug_trybot when cronet's
       # shared library loading is fixed.
       'android_cronet_tester': 'android_cronet_gn_debug_static_bot',
       'android_n5x_swarming_rel': 'swarming_android_gn_release_trybot_arm64',
+      'android_optional_gpu_tests_rel':
+        'swarming_android_gn_release_trybot_arm64',
       'android_swarming_rel': 'swarming_android_gn_release_trybot',
       'cast_shell_android': 'android_cast_gn_debug_static_bot',
       'linux_android_dbg_ng': 'swarming_android_gn_debug_trybot',
       'linux_android_rel_ng': 'swarming_android_gn_release_trybot',
     },
 
     'tryserver.chromium.angle': {
       'android_angle_rel_ng':
         'swarming_gpu_tests_android_gn_release_trybot',
       'linux_angle_chromeos_rel_ng':
@@ skipping 1625 matching lines @@
     },
 
     'drfuzz': { 'gn_args': 'use_drfuzz=true' },
 
     'drmemory': {
       'gn_args': 'is_component_build=true enable_iterator_debugging=false',
       'gyp_defines': 'component=shared_library build_for_tool=drmemory',
     },
   },
 }