Rename testing/buildbot/tryserver.blink to add missing '.json'.

testing/buildbot/tryserver.blink.json

 {
   "android_chromium_gn_compile_rel": {
     "additional_compile_targets": [
       "chrome_shell_apk"
     ],
     "gtest_tests": []
   },
   "linux_chromium_gn_rel": {
     "additional_compile_targets": [
       "all"
     ],
     "gtest_tests": [
       {
         "test": "base_unittests"
       }
     ]
   }
 }