Moves ash content specific tests into ash_content_unittests

testing/buildbot/chromium.memory.json

 {
   "Linux ASan LSan Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 276 matching lines @@
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "app_list_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "ash_content_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "ash_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "aura_unittests"
       },
       {
         "swarming": {
@@ skipping 440 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wtf_unittests"
       }
     ]
   }
 }