Allow a single trybot to mirror multiple waterfall bots.

scripts/slave/recipes/chromium_trybot.expected/analyze_finds_invalid_target.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_rel_ng",
@@ skipping 151 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "get compile targets for scripts",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
+      "python",
+      "-u",
+      "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
+      "[SLAVE_BUILD]/src/testing/buildbot/chromium.gpu.json",
+      "/path/to/tmp/json"
+    ],
+    "cwd": "[SLAVE_BUILD]",
+    "name": "read test spec (chromium.gpu)",
+    "~followup_annotations": [
+      "@@@STEP_TEXT@path: [SLAVE_BUILD]/src/testing/buildbot/chromium.gpu.json@@@",
+      "@@@STEP_LOG_LINE@json.output@{}@@@",
+      "@@@STEP_LOG_END@json.output@@@"
+    ]
+  },
+  {
+    "cmd": [
       "killall",
       "-9",
       "gnome-keyring-daemon"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "killall gnome-keyring-daemon"
   },
   {
     "cmd": [
       "git",
@@ skipping 63 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@failure_hash@\"3908b365fae78e6b96f5e3b38db5b33131cde606\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Error, following targets were not found: invalid target, another one",
     "status_code": 1
   }
 ]