AppListController refactoring part 1: AppListShower implementation.

testing/buildbot/chromium_trybot.json

 {
   "compile_targets": [
     "all"
   ],
   "filter_compile_builders": [
     "win_chromium_compile_dbg",
     "win_chromium_rel_swarming",
     "win_chromium_x64_rel_swarming",
     "win8_chromium_rel"
   ],
   "gtest_tests": [
     {
       "swarming": {
         "can_use_on_swarming_builders": true
       },
       "test": "accessibility_unittests"
     },
     {
       "swarming": {
         "can_use_on_swarming_builders": true
       },
       "test": "app_list_unittests"
     },
     {
+      "swarming": {
+        "can_use_on_swarming_builders": true
+      },
+      "test": "app_list_shower_unittests"
+    },
+    {
       "chromium_configs": [
         "chromium_chromeos",
         "chromium_chromeos_clang",
         "chromium_chromeos_ozone"
       ],
       "swarming": {
         "can_use_on_swarming_builders": true
       },
       "test": "ash_unittests"
     },
@@ skipping 363 matching lines @@
       ],
       "platforms": [
         "linux"
       ],
       "test": "ozone_unittests"
     }
   ],
   "non_filter_builders": [],
   "non_filter_tests_builders": []
 }