Add support for "additional_mirrors" property in trybot configs.

scripts/slave/recipe_modules/chromium_tests/chromium_gpu.py

+# 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.
+
+from . import steps
+
+SPEC = {
+  'settings': {
+    'build_gs_bucket': 'chromium-gpu-fyi-archive',
+    # WARNING: src-side runtest.py is only tested with chromium CQ builders.
+    # Usage not covered by chromium CQ is not supported and can break
+    # without notice.
+    'src_side_runtest_py': True,
+  },
+  'builders': {
+    'GPU Linux Builder': {
+      'chromium_config': 'chromium',
+      'chromium_apply_config': ['mb', 'ninja_confirm_noop',
+                                'archive_gpu_tests', 'chrome_with_codecs',
+                                'internal_gles2_conform_tests'],
+      'gclient_config': 'chromium',
+      'gclient_apply_config': ['chrome_internal'],
+      'chromium_config_kwargs': {
+        'BUILD_CONFIG': 'Release',
+        'TARGET_BITS': 64,
+      },
+      'bot_type': 'builder',
+      'compile_targets': [
+      ],
+      'testing': {
+        'platform': 'linux',
+      },
+      'use_isolate': True,
+      'enable_swarming': True,
+    },
+    'Linux Release (NVIDIA)': {
+      'chromium_config': 'chromium',
+      'chromium_apply_config': ['mb', 'ninja_confirm_noop'],
+      'gclient_config': 'chromium',
+      'chromium_config_kwargs': {
+        'BUILD_CONFIG': 'Release',
+        'TARGET_BITS': 64,
+      },
+      'bot_type': 'tester',
+      'test_generators': [
+        steps.generate_gtest,
+        steps.generate_script,
+        steps.generate_isolated_script,
+      ],
+      'parent_buildername': 'GPU Linux Builder',
+      'testing': {
+        'platform': 'linux',
+      },
+      'enable_swarming': True,
+    },
+  },
+}