Ensure athena_unittests is run for chromeos tryjobs.

testing/buildbot/chromium_trybot.json

 {
   "compile_targets": [
     "all"
   ],
   "gtest_tests": [
     "accessibility_unittests",
     "app_list_unittests",
     {
       "test": "ash_unittests",
       "chromium_configs": [
@@ skipping 200 matching lines @@
       "chromium_configs": [
         "chromium_chromeos",
         "chromium_chromeos_clang",
         "chromium_chromeos_ozone"
       ]
     },
     {
       "test": "athena_unittests",
       "platforms": ["linux"],
       "chromium_configs": [
+        "chromium_chromeos",

[Mr4D (OOO till 08-26), 2014/10/16 17:17:25]

Before I removed it we also used to check clang. I suppose we want to keep that
as well?

         "chromium_chromeos_athena",
         "chromium_chromeos_ozone"
       ]
     },
     {
       "test": "display_unittests",
       "platforms": ["linux"]
     },
     {
       "test": "ozone_unittests",
       "platforms": ["linux"],
       "chromium_configs": ["chromium_chromeos_ozone"]
     }
   ],
 
   "non_filter_builders": [
   ],
 
   "non_filter_tests_builders": [
   ],
 
   "filter_compile_builders": [
     "win_chromium_compile_dbg",
     "win_chromium_rel_swarming",
     "win_chromium_x64_rel_swarming",
     "win8_chromium_rel"
   ]
 }